[BUGFIX] Tests: Better mocking in ext:core

Some tests still rely on initialized $GLOBALS['TYPO3_DB']. The patch
handles some easy cases in ext:core.

Change-Id: I5b720154035cc5f8b156a9358806673daaf9c9ec
Resolves: #56918
Related: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/28394
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring