[BUGFIX] Incomplete backup in AbstractUserAuthenticationTest

The test for issue #50913 exchanged TYPO3_DB with a mock. In 6.0,
this global must be backed up manually.

Change-Id: I7aea5646cd6ce157d86930857e3c6a306f9cfd4f
Resolves: #52346
Related: #50913
Releases: 6.0
Reviewed-on: https://review.typo3.org/24119
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn