test(service-worker): improve adding clients in SwTestHarness (#23625)

This commits changes how clients are added in `SwTestHarness`, so that
the behavior in tests closer mimics what would happen in an actual
ServiceWorker.
It also removes auto-adding clients when calling `clients.get()`, which
could hide bugs related to non-existing clients.

PR Close #23625

packages/service-worker/worker/test/happy_spec.ts

@@ -470,7 +470,6 @@ const manifestUpdateHash = sha1(JSON.stringify(manifestUpdate));
     async_it('shows notifications for push notifications', async() => {
       expect(await makeRequest(scope, '/foo.txt')).toEqual('this is foo');
       await driver.initialized;
-      scope.clients.add('default');
       await scope.handlePush({
         notification: {
           title: 'This is a test',
@@ -726,7 +725,6 @@ async function makeRequest(
   const [resPromise, done] = scope.handleFetch(new MockRequest(url, init), clientId);
   await done;
   const res = await resPromise;
-  scope.clients.add(clientId);
   if (res !== undefined && res.ok) {
     return res.text();
   }