Fix parallel testing (denoland/deno_std#309)

Fixes denoland/deno_std#308

Co-authored by @chiefbiiko
Original: cac060f388

testing/main.ts

@@ -1,3 +1,11 @@
 // Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
 import { runTests } from "./mod.ts";
-runTests();
+
+async function main() {
+  // Testing entire test suite serially
+  await runTests();
+  // Testing parallel execution on a subset that does not depend on exec order
+  await runTests({ parallel: true, only: /^testing/ });
+}
+
+main();