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

explorer/testdata/string/fail_block_quotes_not_on_own_line.carbon

@@ -7,7 +7,7 @@
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:STDERR: SYNTAX ERROR: explorer/testdata/string/fail_block_quotes_not_on_own_line.carbon:[[@LINE+1]]: Invalid block string: Should end with triple quotes: error: closing '''
+  // CHECK:STDERR: SYNTAX ERROR: fail_block_quotes_not_on_own_line.carbon:[[@LINE+1]]: Invalid block string: Should end with triple quotes: error: closing '''
   var s: String = '''
     error: closing ''' is not on its own line.
   ''';

explorer/testdata/string/fail_hex_lower.carbon

@@ -7,7 +7,7 @@
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:STDERR: SYNTAX ERROR: explorer/testdata/string/fail_hex_lower.carbon:[[@LINE+1]]: Invalid escaping in string: "str\xaa"
+  // CHECK:STDERR: SYNTAX ERROR: fail_hex_lower.carbon:[[@LINE+1]]: Invalid escaping in string: "str\xaa"
   Print("str\xaa");
   return 0;
 }

explorer/testdata/string/fail_hex_truncated.carbon

@@ -7,7 +7,7 @@
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:STDERR: SYNTAX ERROR: explorer/testdata/string/fail_hex_truncated.carbon:[[@LINE+1]]: Invalid escaping in string: "str\x"
+  // CHECK:STDERR: SYNTAX ERROR: fail_hex_truncated.carbon:[[@LINE+1]]: Invalid escaping in string: "str\x"
   Print("str\x");
   return 0;
 }

explorer/testdata/string/fail_invalid_escape.carbon

@@ -7,7 +7,7 @@
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:STDERR: SYNTAX ERROR: explorer/testdata/string/fail_invalid_escape.carbon:[[@LINE+1]]: Invalid escaping in string: "str\e"
+  // CHECK:STDERR: SYNTAX ERROR: fail_invalid_escape.carbon:[[@LINE+1]]: Invalid escaping in string: "str\e"
   Print("str\e");
   return 0;
 }

explorer/testdata/string/fail_newline.carbon

@@ -7,7 +7,7 @@
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:STDERR: SYNTAX ERROR: explorer/testdata/string/fail_newline.carbon:[[@LINE+2]]: missing closing quote in single-line string: "new
+  // CHECK:STDERR: SYNTAX ERROR: fail_newline.carbon:[[@LINE+2]]: missing closing quote in single-line string: "new
   // CHECK:STDERR:
   Print("new
 line");

explorer/testdata/string/fail_octal.carbon

@@ -7,7 +7,7 @@
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:STDERR: SYNTAX ERROR: explorer/testdata/string/fail_octal.carbon:[[@LINE+1]]: Invalid escaping in string: "str\01"
+  // CHECK:STDERR: SYNTAX ERROR: fail_octal.carbon:[[@LINE+1]]: Invalid escaping in string: "str\01"
   Print("str\01");
   return 0;
 }

explorer/testdata/string/fail_raw_block_more_hash_tags_on_left.carbon

@@ -7,7 +7,7 @@
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:STDERR: SYNTAX ERROR: explorer/testdata/string/fail_raw_block_more_hash_tags_on_left.carbon:[[@LINE+1]]: Unexpected end of file
+  // CHECK:STDERR: SYNTAX ERROR: fail_raw_block_more_hash_tags_on_left.carbon:[[@LINE+1]]: Unexpected end of file
   var s: String = ##'''
     error: there are more #s on the left than the right.
   '''#;

explorer/testdata/string/fail_raw_block_more_hash_tags_on_right.carbon

@@ -7,7 +7,7 @@
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:STDERR: SYNTAX ERROR: explorer/testdata/string/fail_raw_block_more_hash_tags_on_right.carbon:[[@LINE+1]]: invalid character '\x23' in source file.
+  // CHECK:STDERR: SYNTAX ERROR: fail_raw_block_more_hash_tags_on_right.carbon:[[@LINE+1]]: invalid character '\x23' in source file.
   var s: String = #'''
     error: there are more #s on the right than the left.
   '''##;

explorer/testdata/string/fail_raw_block_quotes_not_on_own_line.carbon

@@ -7,7 +7,7 @@
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:STDERR: SYNTAX ERROR: explorer/testdata/string/fail_raw_block_quotes_not_on_own_line.carbon:[[@LINE+1]]: Invalid block string: Should end with triple quotes: error: closing '''
+  // CHECK:STDERR: SYNTAX ERROR: fail_raw_block_quotes_not_on_own_line.carbon:[[@LINE+1]]: Invalid block string: Should end with triple quotes: error: closing '''
   var s: String = #'''
     error: closing '''# is not on its own line.
   '''#;

explorer/testdata/string/fail_raw_block_single_line.carbon

@@ -7,7 +7,7 @@
 package ExplorerTest api;
 
 fn CompareStr(s: String) -> i32 {
-  // CHECK:STDERR: SYNTAX ERROR: explorer/testdata/string/fail_raw_block_single_line.carbon:[[@LINE+1]]: Invalid block string: Too few lines
+  // CHECK:STDERR: SYNTAX ERROR: fail_raw_block_single_line.carbon:[[@LINE+1]]: Invalid block string: Too few lines
   if (s == #'''raw string literal starting with '''#) {
     return 0;
   }

explorer/testdata/string/fail_raw_more_hash_tags_on_left.carbon

@@ -7,7 +7,7 @@
 package ExplorerTest api;
 
 fn CompareStr(s: String) -> i32 {
-  // CHECK:STDERR: SYNTAX ERROR: explorer/testdata/string/fail_raw_more_hash_tags_on_left.carbon:[[@LINE+2]]: missing closing quote in single-line string: ##"str"#) {
+  // CHECK:STDERR: SYNTAX ERROR: fail_raw_more_hash_tags_on_left.carbon:[[@LINE+2]]: missing closing quote in single-line string: ##"str"#) {
   // CHECK:STDERR:
   if (s == ##"str"#) {
     return 0;

explorer/testdata/string/fail_raw_more_hash_tags_on_right.carbon

@@ -7,7 +7,7 @@
 package ExplorerTest api;
 
 fn CompareStr(s: String) -> i32 {
-  // CHECK:STDERR: SYNTAX ERROR: explorer/testdata/string/fail_raw_more_hash_tags_on_right.carbon:[[@LINE+1]]: invalid character '\x23' in source file.
+  // CHECK:STDERR: SYNTAX ERROR: fail_raw_more_hash_tags_on_right.carbon:[[@LINE+1]]: invalid character '\x23' in source file.
   if (s == "str"#) {
     return 0;
   }

explorer/testdata/string/fail_tab.carbon

@@ -7,7 +7,7 @@
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:STDERR: SYNTAX ERROR: explorer/testdata/string/fail_tab.carbon:[[@LINE+1]]: Invalid escaping in string: "new	line"
+  // CHECK:STDERR: SYNTAX ERROR: fail_tab.carbon:[[@LINE+1]]: Invalid escaping in string: "new	line"
   Print("new	line");
   return 0;
 }