[TASK] Adapt DriverRegistryTest to the changed behaviour

The DriverRegistry does not throw exception anymore, if the
same Driver is registered twice. The Exception only is
thrown if another driver takes the same key.

Change-Id: I16c0aa5bc98d7da90a47615ea675ab655391e1dc
Releases: 6.0
Fixes: 40843
Reviewed-on: http://review.typo3.org/14669
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter