Convert CommentDefaultFormatterCacheTagsTest from web test to entity unit test

comment-unit-test.patch queued for re-testing.

+++ b/core/modules/comment/lib/Drupal/comment/Tests/CommentDefaultFormatterCacheTagsTest.php
@@ -39,9 +40,17 @@ public static function getInfo() {
-    $this->drupalLogin($this->drupalCreateUser(array(
-      'access comments',
-    )));
+    // Set the current user to uid=1 to bypass all permission checks, and
+    // therefore, render all comments.
+    $this->container->set('current_user', new UserSession(array('uid' => 1)));

No!

This must explicitly remain a user with only the 'access comments' permission, for the reasons outlined in #2099131-201: Use #pre_render pattern for entity render caching.

#7: It says "published comment" in the code comment because that's what CommentDefaultFormatter's if-test says. But the reason I'm insisting on using 'access comments' is because the condition to render comments that applies to 99% of users (who don't have the permission to administer comments) looks like this:
(($entity->get($field_name)->comment_count && $this->currentUser->hasPermission('access comments'))
And in my work on #2099131: Use #pre_render pattern for entity render caching, I discovered that $comment->save() did NOT reset the commented entity's storage cache, which caused comment_count to remain at zero, even after comments were posted.

Giving users all permissions tests the 1% case, giving the user only permission to access comments tests the 99% case.

Hopefully this is more clear. But I know, it's confusing and frustratingly complex. Again, see #2099131-201: Use #pre_render pattern for entity render caching for all the details, and an interdiff that shows the problems explicitly.

Agreed.

The key problem is that CommentDefaultFormatter was added with ZERO test coverage. I only added the regression test coverage needed to prevent regressions of the bugs I fixed and discovered while working on something completely independent.

Committed/pushed to 8.x, thanks!

Mh, sadly this test does not really work as you would expect it and causes kind of "random" failures on other patches, see #2263329: CommentDefaultFormatterCacheTagsTest just works because nothing uses AccountProxyInterface in HEAD

Ha, effulgentsia should have know that better.