RuntimeTest: adding getHookRequirementsSeverity() to assure flexibility for ourselves, as core might change the constant values or any other trickery we should be able to quickly act upon.

RuntimeTestService now also has a getHookRequirementsArray() helper of which its output can be straight returned from hook_requirements(), nice!

RuntimeTestService: as direct access to the tests is (sometimes) preferable and not really dangerous in any way, I implemented Iterable and Countable #tested.

The first attempt at a serious RuntimeTest API that allows submodules to raise problems and verify system stability. Makes it lots easier for - for instance - a Varnish implementation to check its settings, poll the server and inspect whether the queue isn't going bonkers.

Renaming Diagnostic/ to RuntimeTest/ as the tests are clearly ran at runtime but not necessarily always of diagnostic nature.

PurgeQueueTestBase: wrote this very thorough base tests which has clean derivatives for the Database, File and Memory queue. Although all tests pass perfectly on File and Memory, there's a weird bug in PDO occuring only on the Database plugin, which hasn't been solved yet:

Drupal\Core\Database\DatabaseExceptionWrapper: SQLSTATE[HY000]: General error: failed to instantiate user-supplied statement class: DELETE FROM {queue} WHERE (name = :db_condition_placeholder_0) ; Array ( [:db_condition_placeholder_0] => purge ) in Drupal\Core\Database\Connection->query()

The Memory queue moved back into purge, and I rewrote it entirely by refactoring buffer code out of the File queue. Now the File queue is based on our own Memory queue and therefore makes two queues function 100% reliable when it comes to lease time management of times. WIN!

purge_ui: its form got renamed to PurgeUiMainConfigForm and I wrote a very thorough PurgeUiMainConfigFormTest.

Introducing the purge_test module, which ships with three dummy purgers, three dummy queue's and the memory queue from purgetest. The latter module will slowly dissapear.

Renamed PurgeablesService --> PurgeableService, although the plural was more correct it killed consistency caused confusion.

The purge_core_invalidation submodule is a 100% working fact. Once enabled it will capture and flatten tags that are getting cleared by Drupal, and will queue those at the end of each request.

Switched the queue committing in QueueService by changing from a shutdown function into a proper destructor, which seems to call it reliably.

Adding the purge_cachetags stub module, where parts of https://gist.github.com/Berdir/3fe65aa3b7ad86e16a5e will reside.