[BUGFIX] Database_TestCase incompatible with TYPO3 6.x

With TYPO3 6.1, importStdDb() does not work as intended, as
t3lib/stddb/tables.sql does not exist anymore causing the test
database to be empty.

Resolves: #54898
Change-Id: If48c4121c8aaec6d0778291f40d7990afc97b3f2
Reviewed-on: https://review.typo3.org/26743
Reviewed-by: Oliver Klee
Tested-by: Oliver Klee