[TASK] Adjust namespacing in Extbase-testcases

Classes created for testing should run inside
separate testing-namespace and not pollute
the global namespace.
We also need to extend from \\TYPO3\\...
instead of just TYPO3\\... then.

Change-Id: I0577327a3b3a7ee1be089bf381b153dd1a1a5602
Resolves: #42984
Releases: 6.2
Reviewed-on: https://review.typo3.org/22232
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn