[BUGFIX] Use the TestFinder to determine fixture path

Two tests still use a hardcoded path to the fixture files.
This fails if no typo3_src folder is present which is not
the case in some TYPO3 installations.

For this change it is necessary to also allow tests/ as prefix
for language files.

Change-Id: I89de174c8a80ea162ec3b0c3eef4817e7c8be475
Releases: 6.0
Resolves: #38349
Reviewed-on: http://review.typo3.org/12332
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel