[BUGFIX] Follow up: Incomplete mocking in Tx_Phpunit_BackEnd_ModuleTest

Move the call to purgeInstances for 4.x inside the else branch.

Fixes: #59960
Change-Id: Ie5c304cea16de03c44e0526110c89a6cfd9c5616
Reviewed-on: https://review.typo3.org/31186
Reviewed-by: Oliver Klee
Tested-by: Oliver Klee