Oak Segment Tar tests should not check node store fixture

Checking the node store fixture commons.FixturesHelper#getFixtures() is a left over from when the segment node store was part of oak-core and we wanted to avoid running the tests multiple times. As we are now in a separate module this check is not necessary any more. It is currently even harmful as certain tests are skipped. The default value for the fixtures is still SEGMENT_MK because oak-segment-tar cannot yet depend on Oak 1.5.9 (not released yet) where the default was switched to SEGMENT_TAR (see OAK-4706).