tools: Fix test-runner --filter flag on windows

Likely got broken with https://dawn-review.googlesource.com/c/tint/+/56776

Change-Id: Ie9db21e11084435a47f6fa0bfc809a590a353685
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/58396
Auto-Submit: Ben Clayton <bclayton@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>

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

@@ -116,8 +116,7 @@ func run() error {
 	// Split the --filter flag up by ',', trimming any whitespace at the start and end
 	globIncludes := strings.Split(filter, ",")
 	for i, s := range globIncludes {
-		// Escape backslashes for the glob config
-		s = strings.ReplaceAll(s, `\`, `\\`)
+		s = filepath.ToSlash(s) // Replace '\' with '/'
 		globIncludes[i] = `"` + strings.TrimSpace(s) + `"`
 	}