[BUGFIX] Correct naming of TYPO3 database backend test

Class and filename are called DbBackendTest, but should be
Typo3DatabaseBackendTest.

Change-Id: I1ae8fca733151e05ae4aedf8a5a479f568e3e998
Fixes: #47082
Releases: 6.1, 6.0
Reviewed-on: https://review.typo3.org/19765
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn