[TASK] Tests: Inherit from UnitTestCase in ext:fluid

Changes class inheritance from ext:exbase BaseTestCase
to ext:core UnitTestCase in ext:fluid

Change-Id: I46667d9cb5a56f5bc29c4f2c3525c6556d5f802f
Resolves: #56958
Related: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/28440
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn