[TASK] Prepare Frontend Tests for additional site handling tests

Some test functionality regarding Site Handling is now sorted out in order
to add more tests.

- SiteConfiguration setup for tests is moved into a separate Trait.
- Code which is also used (in the future) for EXT:core and not
   just frontend is moved to EXT:core
- LocalizedFrontendPages now have a slug field as well.
- Regular Frontend JsonRenderer now also renders values
  from SiteLanguage config + context aspect

Resolves: #88060
Releases: master, 9.5
Change-Id: If859da89e4fcec16e3d1c2abb934eef040d4f386
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/60363


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>