[BUGFIX] Backup global vars in DriverRegistryTest

The DriverRegistryTest pollutes the global variable space,
which leads to failures in other tests if they are executed
after this test.

Backup and restore the global variables for this test.

Change-Id: I1abacbb8a49a771c64356215749c96ae02700cc2
Resolves: #38350
Releases: 6.0
Reviewed-on: http://review.typo3.org/12333
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel