[BUGFIX] Follow-Up to #41828 - Incomplete mocking in tests

Better mocking in test class to ensure isUsageEnabled() returns TRUE
in all environments and especially in travis.

Change-Id: Icb68b1c863c13db0fb1e48390db130f43b9e0622
Related: #41828
Releases: 6.0
Reviewed-on: http://review.typo3.org/15815
Reviewed-by: Steffen Ritter
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn