[BUGFIX] Tests: Improve mocking in ext:belog

The subject method initialize() calls some static dependencies
that can not be mocked. They do DB actions in the end. The patch
now mocks TYPO3_DB to suppress this DB call.

Change-Id: I43b0e002bfab081ed41a5c259011f465b507a74b
Resolves: #56718
Related: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/28215
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn