TASK: Deprecate BaseTestCase functionalities

[mhsdesign]

Flow has a custom extension of php unit mocks, that comes from times of typo3. Via the accessibly proxy it allows for calling even protected methods and access of protected properties.

The usage and retrieval BaseTestCase::getAccessibleMock() has been deprecated as one should not use this for testing. It will lead to a super high coupling of the tests to the super internal implementation which makes refactoring basically impossible (without rewriting major parts of the tests). In Neos.Neos we see this very well because we are just removing these tests and rewriting them in behat for Neos 9 :D.

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Checklist

• [ ] Code follows the PSR-2 coding style
• [ ] Tests have been created, run and adjusted as needed
• [ ] The PR is created against the lowest maintained branch
• [ ] Reviewer - PR Title is brief but complete and starts with FEATURE|TASK|BUGFIX
• [ ] Reviewer - The first section explains the change briefly for change-logs
• [ ] Reviewer - Breaking Changes are marked with !!! and have upgrade-instructions