[TASK] Reduce memory footprint of unit test suite

Switching to transientMemoryBackend for some caches raises the
memory footprint of unit test. Change the bootstrap a bit to not
use this cache backend in unit tests.

Change-Id: I3bc4774c4c9c6a24314681491bfdf23a9eab570d
Resolves: #56585
Related: #56583
Releases: 6.2
Reviewed-on: https://review.typo3.org/28089
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel