Sort out ApcuBackendTest

Problem/Motivation

ApcuBackendTest has both @requires and odd \Drupal\KernelTests\Core\Cache\ApcuBackendTest::getRequirements / \Drupal\KernelTests\Core\Cache\ApcuBackendTest::requirementsFail code.

I think now we have @requires extension apcu we can remove the requirements code because it'll never fail.

Proposed resolution

1. Remove \Drupal\KernelTests\Core\Cache\ApcuBackendTest::getRequirements() and \Drupal\KernelTests\Core\Cache\ApcuBackendTest::requirementsFail()
2. Remove all calls to those methods and remove overrides of tests where possible - for example testDelete no longer needs an override.

Remaining tasks

User interface changes

None

API changes

None

Data model changes

None

Release notes snippet

N/a