[BUGFIX] Incomplete mocking in Tx_Phpunit_BackEnd_ModuleTest

The singleton instances needs to be saved an resetted to avoid side
effects while running the tests.

Fixes: #59960
Change-Id: I07e8f954f81397654c02e67c25202dcbe1c5391f
Reviewed-on: https://review.typo3.org/31184
Reviewed-by: Oliver Klee
Tested-by: Oliver Klee