[BUGFIX] Adopt TestFinderTest to 6.2

Skip test that fail due to the new package manager approach of the core.

Add a test that checks for the extension key 'phpunit' across all
versions, to ensure that the extensions list can be read.

Fixes: #59962
Change-Id: I5631093b921106efec07313ecaead2bb7d44b95d
Reviewed-on: https://review.typo3.org/31189
Reviewed-by: Oliver Klee
Tested-by: Oliver Klee