Replace MemoryCounterBackend with PHPUnit mocks

Problem/Motivation

\Drupal\Core\Cache\MemoryCounterBackend is only used in tests and has the following @todo:

 * @todo On the longrun this backend should be replaced by phpunit mock objects.

It is only used in two tests:

• Drupal\Tests\migrate\Kernel\MigrateSqlSourceTestBase
• Drupal\Tests\path_alias\Kernel\AliasTest

Steps to reproduce

Proposed resolution

Refactor the tests to use PHPUnit mocks instead.
Deprecate MemoryCounterBackend for removal.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet