[TASK] Add frontend functional tests for pages having slugs

Functional tests for pages having site configuration and using slugs
are introduced. Existing tests (not relying on slugs) are still kept
in addition to the new tests in order to still have tests for legacy
"index.php?id=123" behavior.

Resolves: #85962
Releases: master
Change-Id: I7902bfd028f9c8a1d41f3573d72a2a7e96148b43
Reviewed-on: https://review.typo3.org/58015


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Tested-by: Frans Saris <franssaris@gmail.com>