Core Tests: have a check for duplicate markers in inc files

[jrfnl]

A number of tests in the Core test directory identify a specific token via a /* testMarker */ like comment in the inc file(s).

When the test case file inc gets larger, it is quite easy to accidentally have a duplicate "test marker" comment in the file (non unique name) and the test would end up not testing what it should be testing and this would go unnoticed.

It would be good to have a minimal test - probably with a helper function running the actual test code in the AbstractMethodUnitTest class - to verify that an inc file doesn't contain duplicate test markers.

This would be a QA check on the tests, but can be run as a test itself.

[jrfnl]

Note: probably also good to have this available via PHPCSUtils as well.