[TASK] Remove loadExtensionTables() from unit test bootstrap

With proper mocking in place, loadExtensionTables() can be
removed from unit test bootstrap.
This speeds up the test suite quite a bit since the huge
$GLOBALS['TCA'] is no longer initialized and gives backup code
of phpunit less headaches.

Change-Id: I9ac96047a49914f3c6ab7acc318bbc36488b6c5b
Resolves: #56739
Related: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/28235
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring