[TASK] Inherit from UnitTestCase in ext:extbase

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

Change-Id: If2d2afbc05765b3ff1d8fa3fded17e6fb0eecd44
Resolves: #56961
Related: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/28441
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters