ServiceWorker: Add t.add_cleanup() in WPT tests.

This CL adds t.add_cleanup() that calls service_worker_unregister()
in WPT tests to ensure registrations are removed on test completion.

Bug: 824647
Change-Id: Ieb790d4d7f63fc4810f6062d7e288aad1e88f45d
Reviewed-on: https://chromium-review.googlesource.com/c/1328545
Commit-Queue: Hiroki Nakagawa <[email protected]>
Reviewed-by: Hiroki Nakagawa <[email protected]>
Reviewed-by: Matt Falkenhagen <[email protected]>
Cr-Commit-Position: refs/heads/master@{#608693}

service-workers/service-worker/import-module-scripts.https.html

@@ -13,6 +13,7 @@
       navigator.serviceWorker.onmessage = resolve;
     });
     await service_worker_unregister(t, testCase.scriptURL);
+    t.add_cleanup(() => service_worker_unregister(t, testCase.scriptURL));
     const registration = await navigator.serviceWorker.register(
       testCase.scriptURL,
       { scope: testCase.scriptURL, type: 'module' });