Provide local paths for file tests. (#2830)

The intent of this change is that instead of paths looking like `explorer/testdata/foo/bar.carbon` (repo-relative), they're now just `bar.carbon` (local). The consequence is that paths should be a bit more durable in various environments, and just paths should be shorter and easier to read.

The explorer's prelude is an exception to this since it comes from data, rather than being the test target. Due to the change in approaches, it needs the regex again.

Uses #2829

toolchain/parser/testdata/function/declaration/fail_identifier_instead_of_sig.carbon

@@ -10,5 +10,5 @@
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 
-// CHECK:STDERR: toolchain/parser/testdata/function/declaration/fail_identifier_instead_of_sig.carbon:[[@LINE+1]]:8: `fn` requires a `(` for parameters.
+// CHECK:STDERR: fail_identifier_instead_of_sig.carbon:[[@LINE+1]]:8: `fn` requires a `(` for parameters.
 fn foo bar;

toolchain/parser/testdata/function/declaration/fail_missing_deduced_close.carbon

@@ -16,7 +16,7 @@
 // CHECK:STDOUT: ]
 
 // Fix and uncomment this to test error handling.
-// CHECK:STDERR: toolchain/parser/testdata/function/declaration/fail_missing_deduced_close.carbon:[[@LINE+2]]:7: Closing symbol does not match most recent opening symbol.
-// CHECK:STDERR: toolchain/parser/testdata/function/declaration/fail_missing_deduced_close.carbon:[[@LINE+1]]:8: Expected parameter declaration.
+// CHECK:STDERR: fail_missing_deduced_close.carbon:[[@LINE+2]]:7: Closing symbol does not match most recent opening symbol.
+// CHECK:STDERR: fail_missing_deduced_close.carbon:[[@LINE+1]]:8: Expected parameter declaration.
 fn Div[();
-// CHECK:STDERR: toolchain/parser/testdata/function/declaration/fail_missing_deduced_close.carbon:[[@LINE+0]]:174: A `(` for parameters is required after deduced parameters.
+// CHECK:STDERR: fail_missing_deduced_close.carbon:[[@LINE+0]]:127: A `(` for parameters is required after deduced parameters.

toolchain/parser/testdata/function/declaration/fail_missing_name.carbon

@@ -9,5 +9,5 @@
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 
-// CHECK:STDERR: toolchain/parser/testdata/function/declaration/fail_missing_name.carbon:[[@LINE+1]]:4: `fn` introducer should be followed by a name.
+// CHECK:STDERR: fail_missing_name.carbon:[[@LINE+1]]:4: `fn` introducer should be followed by a name.
 fn ();

toolchain/parser/testdata/function/declaration/fail_no_sig_or_semi.carbon

@@ -10,5 +10,5 @@
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 
-// CHECK:STDERR: toolchain/parser/testdata/function/declaration/fail_no_sig_or_semi.carbon:[[@LINE+1]]:7: `fn` requires a `(` for parameters.
+// CHECK:STDERR: fail_no_sig_or_semi.carbon:[[@LINE+1]]:7: `fn` requires a `(` for parameters.
 fn foo

toolchain/parser/testdata/function/declaration/fail_only_fn_and_semi.carbon

@@ -9,5 +9,5 @@
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 
-// CHECK:STDERR: toolchain/parser/testdata/function/declaration/fail_only_fn_and_semi.carbon:[[@LINE+1]]:3: `fn` introducer should be followed by a name.
+// CHECK:STDERR: fail_only_fn_and_semi.carbon:[[@LINE+1]]:3: `fn` introducer should be followed by a name.
 fn;

toolchain/parser/testdata/function/declaration/fail_repeated_fn_and_semi.carbon

@@ -9,5 +9,5 @@
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 
-// CHECK:STDERR: toolchain/parser/testdata/function/declaration/fail_repeated_fn_and_semi.carbon:[[@LINE+1]]:4: `fn` introducer should be followed by a name.
+// CHECK:STDERR: fail_repeated_fn_and_semi.carbon:[[@LINE+1]]:4: `fn` introducer should be followed by a name.
 fn fn;

toolchain/parser/testdata/function/declaration/fail_skip_indented_newline_until_outdent.carbon

@@ -14,7 +14,7 @@
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 
-  // CHECK:STDERR: toolchain/parser/testdata/function/declaration/fail_skip_indented_newline_until_outdent.carbon:[[@LINE+1]]:6: `fn` introducer should be followed by a name.
+  // CHECK:STDERR: fail_skip_indented_newline_until_outdent.carbon:[[@LINE+1]]:6: `fn` introducer should be followed by a name.
   fn (x,
       y,
       z)

toolchain/parser/testdata/function/declaration/fail_skip_indented_newline_with_semi.carbon

@@ -14,7 +14,7 @@
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 
-// CHECK:STDERR: toolchain/parser/testdata/function/declaration/fail_skip_indented_newline_with_semi.carbon:[[@LINE+1]]:4: `fn` introducer should be followed by a name.
+// CHECK:STDERR: fail_skip_indented_newline_with_semi.carbon:[[@LINE+1]]:4: `fn` introducer should be followed by a name.
 fn (x,
     y,
     z);

toolchain/parser/testdata/function/declaration/fail_skip_indented_newline_without_semi.carbon

@@ -14,7 +14,7 @@
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 
-// CHECK:STDERR: toolchain/parser/testdata/function/declaration/fail_skip_indented_newline_without_semi.carbon:[[@LINE+1]]:4: `fn` introducer should be followed by a name.
+// CHECK:STDERR: fail_skip_indented_newline_without_semi.carbon:[[@LINE+1]]:4: `fn` introducer should be followed by a name.
 fn (x,
     y,
     z)

toolchain/parser/testdata/function/declaration/fail_skip_to_newline_without_semi.carbon

@@ -14,6 +14,6 @@
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 
-// CHECK:STDERR: toolchain/parser/testdata/function/declaration/fail_skip_to_newline_without_semi.carbon:[[@LINE+1]]:4: `fn` introducer should be followed by a name.
+// CHECK:STDERR: fail_skip_to_newline_without_semi.carbon:[[@LINE+1]]:4: `fn` introducer should be followed by a name.
 fn ()
 fn F();

toolchain/parser/testdata/function/declaration/fail_skip_without_semi_to_curly.carbon

@@ -13,6 +13,6 @@
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 
-// CHECK:STDERR: toolchain/parser/testdata/function/declaration/fail_skip_without_semi_to_curly.carbon:[[@LINE+1]]:1: Unrecognized declaration introducer.
+// CHECK:STDERR: fail_skip_without_semi_to_curly.carbon:[[@LINE+1]]:1: Unrecognized declaration introducer.
 struct X { fn () }
 fn F();

toolchain/parser/testdata/function/declaration/fail_with_identifier_as_param.carbon

@@ -15,5 +15,5 @@
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 
-// CHECK:STDERR: toolchain/parser/testdata/function/declaration/fail_with_identifier_as_param.carbon:[[@LINE+1]]:11: Expected parameter declaration.
+// CHECK:STDERR: fail_with_identifier_as_param.carbon:[[@LINE+1]]:11: Expected parameter declaration.
 fn foo(bar);

toolchain/parser/testdata/function/declaration/fail_without_name_and_many_tokens_in_params.carbon

@@ -9,5 +9,5 @@
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 
-// CHECK:STDERR: toolchain/parser/testdata/function/declaration/fail_without_name_and_many_tokens_in_params.carbon:[[@LINE+1]]:4: `fn` introducer should be followed by a name.
+// CHECK:STDERR: fail_without_name_and_many_tokens_in_params.carbon:[[@LINE+1]]:4: `fn` introducer should be followed by a name.
 fn (a tokens c d e f g h i j k l m n o p q r s t u v w x y z);