Remove CacheTestBase and merge its functionality into ClearTest

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Rerolled to an MR, snuck in some property and param types and changed one of the public methods to protected. Wonder if this can be a kernel test but not sure it will cover everything that drupal_flush_all_caches does. Updated the issue summary with the standard template.

Seems like a good refactor to me, seems pretty straight forward.

We would normally deprecate test base classes, but for me there's a grey area when modules have them because they're never really supposed to be used outside that module. However, I think it would be good to check in gitlab project search or the contrib grepping site whether there's any modules out there using this before we remove it.

There are 81 instances of CacheTestBase, but most of them look like clones of Drupal core.

Excluding the path core/modules/system there is 1 instance and that is not a code reference but some kind of git log reference. Going to self-restore to RTBC based on this.

Thanks for checking.

Looking at the test now, I don't think it should exist at all, it's a functional test that makes no http requests, looks like it was added in 2012 which when a lot of early test coverage was, and we didn't have real unit tests or kernel tests.

Not sure we need any of this coverage, but we could maybe set a couple of cache items then check they're not there afterrwards in DrupalFlushAllCachesTest?

Some other classes that previously extended CacheTestBase were replaced in #1637478: Add a PHP array cache backend → , not sure why ClearTest wasn't replaced at the same time. We also have \Drupal\KernelTests\Core\Cache\GenericCacheBackendUnitTestBase::testDeleteAll but that doesn't necessarily cover drupal_flush_all_caches.

DrupalFlushAllCachesTest has the @covers ::drupal_flush_all_caches annotation, so that's a good start. But it seems to only be testing that container.modules is updated.

Not sure we need any of this coverage, but we could maybe set a couple of cache items then check they're not there afterrwards in DrupalFlushAllCachesTest?

Yeah we could just move ClearTest::testFlushAllCaches in to DrupalFlushAllCachesTest. Probably that class should test everything that function does, like deleting asset files, twig caches etc. I guess those are all tested independently so it might be hard to find something useful to test.

Unrelated to this issue, but this line in DrupalFlushAllCachesTest is useless now since that function has moved to OOP hooks.
$this->assertFalse(function_exists('system_test_help'));