test-runner: Return non-zero code on failure

Kokoro will use this to fail presubmits on new failures.

Change-Id: I32cf49d0380c7b751ae4410ccd91b20407262e9f
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/51482
Commit-Queue: Ben Clayton <bclayton@chromium.org>
Reviewed-by: James Price <jrprice@google.com>

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

@@ -324,6 +324,10 @@ func run() error {
 	fmt.Println()
 	fmt.Println()
 
+	if numFail > 0 {
+		os.Exit(1)
+	}
+
 	return nil
 }