[TASK] Use 6.2 schema for unit tests

Unit tests are 6.2 compatible, but not anymore older core versions,
using namespace for tests, add a UnitTests.xml

Change-Id: I3bef4b5a7fdad5399992724cf38e7d47a8b7c3b6
Reviewed-on: https://review.typo3.org/30783
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn