[BUGFIX] Tests: Improve mocking in ext:extbase

Unit tests reveal some warnings which are caused by incomplete
mocking. They all belong to $GLOBALS['LANG']

Change-Id: Ic2adda9984a530b82e259c1d8931015996f167ba
Resolves: #56700
Related: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/28407
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters