[TASK] Assert clean reference index after DataHandler tests

To verify DataHandler properly calculates correct reference index
entries, the AbstractDataHandlerActionTestCase is extended to
test in tearDown() whether the reference index is clean. If not,
the according test fails.
This should put an end to broken reference index entries once
fully rolled out.
For now, the main ext:core DataHandler tests come with this
test enabled, the workspace related tests will have dedicated
patches.

Change-Id: I583fb109d24a9eff30966d0d20672552f187b709
Releases: master, 10.4
Resolves: #92467
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/65953


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>