[TASK] Tests: Use existing session

Using Admin->useExistingSession() of Admin actor
instead of Kasper actor speeds up the tests a bit
and should make them more reliable.

Change-Id: Ibb462e87e1ce3725cdc40a7a6afa4e3c668b2b5c
Resolves: #75798
Releases: master
Reviewed-on: https://review.typo3.org/47800


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>