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/array/fail_index.carbon

@@ -8,6 +8,6 @@ package ExplorerTest api;
 
 fn Main() -> i32 {
   var x: [i32; 2] = (0, 1);
-  // CHECK:STDERR: RUNTIME ERROR: explorer/testdata/array/fail_index.carbon:[[@LINE+1]]: index 2 out of range in (0, 1)
+  // CHECK:STDERR: RUNTIME ERROR: fail_index.carbon:[[@LINE+1]]: index 2 out of range in (0, 1)
   return x[2];
 }

explorer/testdata/array/fail_negative_size.carbon

@@ -7,7 +7,7 @@
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:STDERR: COMPILATION ERROR: explorer/testdata/array/fail_negative_size.carbon:[[@LINE+1]]: Array size cannot be negative
+  // CHECK:STDERR: COMPILATION ERROR: fail_negative_size.carbon:[[@LINE+1]]: Array size cannot be negative
   var x: [i32; -1] = ();
   return x[0];
 }

explorer/testdata/array/fail_print_uninitalized_array_element.carbon

@@ -8,7 +8,7 @@ package ExplorerTest impl;
 
 fn Main() -> i32 {
   var my_array : [i32; 1];
-  // CHECK:STDERR: RUNTIME ERROR: explorer/testdata/array/fail_print_uninitalized_array_element.carbon:[[@LINE+1]]: Printing uninitialized value
+  // CHECK:STDERR: RUNTIME ERROR: fail_print_uninitalized_array_element.carbon:[[@LINE+1]]: Printing uninitialized value
   Print("{0}", my_array[0]);
   return 0;
 }

explorer/testdata/array/fail_size_mismatch.carbon

@@ -7,7 +7,7 @@
 package ExplorerTest api;
 
 fn Main() -> i32 {
-  // CHECK:STDERR: COMPILATION ERROR: explorer/testdata/array/fail_size_mismatch.carbon:[[@LINE+1]]: type error in initializer of variable: '(i32, i32, i32)' is not implicitly convertible to '[i32; 2]'
+  // CHECK:STDERR: COMPILATION ERROR: fail_size_mismatch.carbon:[[@LINE+1]]: type error in initializer of variable: '(i32, i32, i32)' is not implicitly convertible to '[i32; 2]'
   var x: [i32; 2] = (0, 1, 2);
   return x[0];
 }