test-runner: on Windows, use forward slash separators in paths passed to tint

This ensures that we don't fail when comparing against expected output
that contains paths in them.

Bug: tint:872
Change-Id: I93d4b940eae2c22175ef3add94bb28f907156ca0
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/53600
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

tools/src/cmd/test-runner/main.go

@@ -380,6 +380,10 @@ func (j job) run(wd, exe, dxcPath, xcrunPath string, generateExpected, generateS
 		file = j.file
 	}
 
+	// Make relative paths use forward slash separators (on Windows) so that paths in tint
+	// output match expected output that contain errors
+	file = strings.ReplaceAll(file, `\`, `/`)
+
 	args := []string{
 		file,
 		"--format", string(j.format),