[BUGFIX] Tests in Localization\Parser\LocallangXmlParserTest fail

If the system is already translated into French two tests in
Localization\Parser\LocallangXmlParserTest fail. This is because another
localization file is taken where test strings are not included.

As the tested functions can't work system independent, the test has to
make sure no system dependencies are used. Therefore the patch
introduces a new language "md5" to prove the workflow.

Resolves: #50870
Releases: 6.2, 6.1, 6.0
Change-Id: I64e8864a7d746c4f5c278f5e67d8dae068d8f920
Reviewed-on: https://review.typo3.org/22918
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind