Automated tests for Announce

A few suggestions on where to look at core for similar tests.

1. You will need a test module that can serve multiple fake json feeds. For example serve a feed with 1 item, have the user look at the feed and then serve another with a new item to ensure the user gets the new item indication.

   See this merge request https://git.drupalcode.org/project/drupal/-/merge_requests/284 because it also deals with a single json feed like this module. You can checkout that branch and then look at:
   core/modules/system/tests/modules/advisory_feed_test: This is a test module that serves the feed. Look at the route advisory_feed_test.json_test in this test module for how it serves the json. The actual json files are in core/modules/system/tests/fixtures/psa_feed

   You may be able to get away with just directly using the fixtures and not having a controller method like\Drupal\advisory_feed_test\Controller\AdvisoryTestController::getPsaJson(). We had to have the controller method there because we need to change the Json files slightly.

2. One difference between the merge request I mention above and the current Announce module is that in the Announce module the URL for retrieving the feed is set in config and the merge request uses a hard-coded URL(see core/modules/system/src/SecurityAdvisories/SecurityAdvisoriesFetcher.php and search for <code>updates.drupal.org/psa.json).

   Using the config URL is easier for testing but it does open the possibility that someone could change the URL to a different json feed. If we only want to support the json feed from drupal.org I would suggest this module also switch to a hard-coded feed URL.

   To test the hard-coded feed the test module uses a service decorator on the http_client service.
   See: AdvisoriesTestHttpClient
   and the service definition http_client.advisory_feed_test
   The service also has helper method setting the endpoint to test: \Drupal\advisory_feed_test\AdvisoriesTestHttpClient::setTestEndpoint()

   You should be able to copy this test module, do some renames and remove the parts you don't need.

3. core/modules/system/tests/src/Functional/SecurityAdvisories/SecurityAdvisoryTest.php is the function test that uses the fake json feed.

   In your case you will probably want this to be FunctionalJavascript test to opening the announcements dialog.

   see core/modules/workspaces/tests/src/FunctionalJavascript/WorkspaceToolbarIntegrationTest.php for example of FunctionalJavascript test that opens the off-canvas dialog that this test is also using

4. You will also need tests for services announce.fetcher and announce.user_status

   This should probably be kernel tests. You can look at core/modules/system/tests/src/Kernel/SecurityAdvisories/SecurityAdvisoriesFetcherTest.php in the same merge request. This is similar to what you will need as it tests the public method of the class with mocked feed items. It uses different method than the functional test to provide fake feed items. See \Drupal\Tests\system\Kernel\SecurityAdvisories\SecurityAdvisoriesFetcherTest::setFeedItems().

The test are looking pretty good!

since these have already been committed I created an MR to just had a review of the tests. I have only started with AlertsJsonFeedTest but I think some of comments will apply to the other classes too.

Let me know if you have questions by leaving MR comments and ping to review the changes

@tedbow, I am merging this feedback PR
I am opening up a new PR of the module against Drupal core for the maintainer's feedback.

I'll share the PR here as well, so you can do another round of review in the main PR itself.

Since the issue is merged, moving this to fixed... Let's work on the core PR if there are any suggestions.