[TASK] Streamline autoloading of test classes in extbase

- Adjust namespaces of classes according to PSR-4
- Add a classmap for test fixtures

Releases: master
Resolves: #89706
Change-Id: Iefd73f8039eb3db4de419ecaab13a18f6abb23e2
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62341


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Jörg Bösche <typo3@joergboesche.de>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: Jörg Bösche <typo3@joergboesche.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>