[BUGFIX] Tests: Incomplete Mocking in ext:extensionmanager

After the removal of GLOBALS initialisation the extension raises
warnings in unittests.

Change-Id: I450932b1de1852302e4ae9861b250c5721f6032e
Resolves: #56930
Related: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/28412
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Thomas Maroschik
Tested-by: Thomas Maroschik