[TASK] Add frontend tests

EXT:styleguide is capable to create
a frontend page with a single click.
Therefor, acceptance tests have been
created to test each content element
for expected classes and text.

Also PHP8 undefined array key warnings
have been fixed to get tests working
as expected.

Resolves: #94649
Releases: master
Change-Id: Ie7586bd01a2b0f02c75fb300cc346c9129c346a4
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70118


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>