[TASK] Better mocking of $GLOBALS['LANG']

To further remove dependencies in unit tests, some tests are
adapted to correctly mock $GLOBALS['LANG']. The PageRenderer
tests still need adaptions in a separate patch, so the test
bootstrap still initializes the language handling for now.

Change-Id: I5d6a8a0a0fe738fef676fbc637a02fe9700ccee3
Resolves: #53476
Related: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/25234
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn