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/semantics/testdata/return/fail_type_mismatch.carbon

@@ -37,6 +37,6 @@
 // CHECK:STDOUT: ]
 
 fn Main() -> i32 {
-  // CHECK:STDERR: toolchain/semantics/testdata/return/fail_type_mismatch.carbon:[[@LINE+1]]:13: Cannot implicitly convert from `f64` to `i32`.
+  // CHECK:STDERR: fail_type_mismatch.carbon:[[@LINE+1]]:13: Cannot implicitly convert from `f64` to `i32`.
   return 1.0;
 }

toolchain/semantics/testdata/return/fail_value_disallowed.carbon

@@ -37,7 +37,7 @@
 // CHECK:STDOUT: ]
 
 fn Main() {
-  // CHECK:STDERR: toolchain/semantics/testdata/return/fail_value_disallowed.carbon:[[@LINE+2]]:11: No return expression should be provided in this context.
-  // CHECK:STDERR: toolchain/semantics/testdata/return/fail_value_disallowed.carbon:[[@LINE-2]]:1: There was no return type provided.
+  // CHECK:STDERR: fail_value_disallowed.carbon:[[@LINE+2]]:11: No return expression should be provided in this context.
+  // CHECK:STDERR: fail_value_disallowed.carbon:[[@LINE-2]]:1: There was no return type provided.
   return 0;
 }

toolchain/semantics/testdata/return/fail_value_missing.carbon

@@ -34,6 +34,6 @@
 // CHECK:STDOUT: ]
 
 fn Main() -> i32 {
-  // CHECK:STDERR: toolchain/semantics/testdata/return/fail_value_missing.carbon:[[@LINE+1]]:9: Must return a i32.
+  // CHECK:STDERR: fail_value_missing.carbon:[[@LINE+1]]:9: Must return a i32.
   return;
 }