[BUGFIX] Allow MountPoints with the same slug and add tests

If a multi-site setup contains a page with slug "/products" as DOKTYPE=7
and "/products" in another site which is the Mounted Page the PageRouter
goes in an endless-loop as there is no context in the recursive call.

In addition, some other changes that have been adapted in the previous
9.5 backport are addressed now.

On top, there is now a MountPointScenario which serves as a basis for adding
more functionality in multi-lingual setups.

The next step is also to remove the redirect of the MountPoint page
with mount_pid_ol=1.

Resolves: #90166
Related: #86331
Releases: master, 9.5
Change-Id: If5c67ac813430f54737192341e22b58c9c275cf6
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63007


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Benni Mack <benni@typo3.org>