[cookie-store] Deflake service worker registration (part 2)

This is a follow-up to
web-platform-tests/wpt#18324

This change by itself isn't sufficient to deflake the two tests.

Author: Hexcles

cookie-store/serviceworker_cookieStore_subscriptions_eventhandler_attribute.tentative.https.html

@@ -11,11 +11,11 @@
 (async () => {
   const scope = 'scope';
 
-  let registration = await navigator.serviceWorker.getRegistration(scope);
-  if (registration)
-    await registration.unregister();
-  registration = await navigator.serviceWorker.register(
+  const registration = await navigator.serviceWorker.register(
       'serviceworker_cookieStore_subscriptions_eventhandler_attribute.js', {scope});
+  add_completion_callback(() => {
+    registration.unregister();
+  });
 
   fetch_tests_from_worker(registration.installing);
 })();

cookie-store/serviceworker_cookieStore_subscriptions_mismatch.tentative.https.html

@@ -10,11 +10,11 @@
 (async () => {
   const scope = 'scope';
 
-  let registration = await navigator.serviceWorker.getRegistration(scope);
-  if (registration)
-    await registration.unregister();
-  registration = await navigator.serviceWorker.register(
+  const registration = await navigator.serviceWorker.register(
       'serviceworker_cookieStore_subscriptions_mismatch.js', {scope});
+  add_completion_callback(() => {
+    registration.unregister();
+  });
 
   fetch_tests_from_worker(registration.installing);
 })();