[TASK] Tests: Reduce unit test bootstrap

The patch further reduces the unit test bootstrap: ext_localconf
of extensions is not loaded anymore and the cache framework is
no longer initialized. A test with incommplete mocking in this area
is fixed along the way.
Furthermore the core exception handling is no longer initialized,
so it does not depend on configuration options. A warning
triggered by a unit test will now reliable let the test fail.

Change-Id: I659441482a19eb6b434d520ecded6df8f5086820
Resolves: #51436
Related: #56967
Releases: 6.2
Reviewed-on: https://review.typo3.org/28449
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring