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/package/fail_extra_string.carbon

@@ -12,5 +12,5 @@
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 
-// CHECK:STDERR: toolchain/parser/testdata/package/fail_extra_string.carbon:[[@LINE+1]]:27: Expected a `api` or `impl`.
+// CHECK:STDERR: fail_extra_string.carbon:[[@LINE+1]]:27: Expected a `api` or `impl`.
 package Foo library "bar" "baz";

toolchain/parser/testdata/package/fail_library_is_identifier.carbon

@@ -10,5 +10,5 @@
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 
-// CHECK:STDERR: toolchain/parser/testdata/package/fail_library_is_identifier.carbon:[[@LINE+1]]:26: Expected a string literal to specify the library name.
+// CHECK:STDERR: fail_library_is_identifier.carbon:[[@LINE+1]]:26: Expected a string literal to specify the library name.
 package Geometry library Shapes api;

toolchain/parser/testdata/package/fail_library_skips_name.carbon

@@ -9,5 +9,5 @@
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 
-// CHECK:STDERR: toolchain/parser/testdata/package/fail_library_skips_name.carbon:[[@LINE+1]]:9: Expected identifier after `package`.
+// CHECK:STDERR: fail_library_skips_name.carbon:[[@LINE+1]]:9: Expected identifier after `package`.
 package library "Shapes" api;

toolchain/parser/testdata/package/fail_name_is_keyword.carbon

@@ -9,5 +9,5 @@
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 
-// CHECK:STDERR: toolchain/parser/testdata/package/fail_name_is_keyword.carbon:[[@LINE+1]]:9: Expected identifier after `package`.
+// CHECK:STDERR: fail_name_is_keyword.carbon:[[@LINE+1]]:9: Expected identifier after `package`.
 package fn;

toolchain/parser/testdata/package/fail_no_name.carbon

@@ -9,5 +9,5 @@
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 
-// CHECK:STDERR: toolchain/parser/testdata/package/fail_no_name.carbon:[[@LINE+1]]:8: Expected identifier after `package`.
+// CHECK:STDERR: fail_no_name.carbon:[[@LINE+1]]:8: Expected identifier after `package`.
 package;

toolchain/parser/testdata/package/fail_no_semi.carbon

@@ -11,5 +11,5 @@
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 
-// CHECK:STDERR: toolchain/parser/testdata/package/fail_no_semi.carbon:[[@LINE+1]]:21: Expected `;` to end package directive.
+// CHECK:STDERR: fail_no_semi.carbon:[[@LINE+1]]:21: Expected `;` to end package directive.
 package Geometry api

toolchain/parser/testdata/package/fail_no_type.carbon

@@ -10,5 +10,5 @@
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 
-// CHECK:STDERR: toolchain/parser/testdata/package/fail_no_type.carbon:[[@LINE+1]]:17: Expected a `api` or `impl`.
+// CHECK:STDERR: fail_no_type.carbon:[[@LINE+1]]:17: Expected a `api` or `impl`.
 package Geometry;

toolchain/parser/testdata/package/fail_omit_library_keyword.carbon

@@ -10,5 +10,5 @@
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
 
-// CHECK:STDERR: toolchain/parser/testdata/package/fail_omit_library_keyword.carbon:[[@LINE+1]]:18: Missing `library` keyword.
+// CHECK:STDERR: fail_omit_library_keyword.carbon:[[@LINE+1]]:18: Missing `library` keyword.
 package Geometry "Shapes" api;