[BUGFIX] Remaining PHP 8.0 fixes for acceptance tests

This fixes a bunch of PHP 8.0 warnings found by acceptance
testing. image processing and ext:scheduler are affected
for the main part.
This makes the ac test suite green with PHP 8.0. The
gitlab activation and another testing-framework raise
will follow with dedicated patches.
The patch brings an additional acceptance test for
install tool 'Environment->Image Processing' since
this worked well to nail lots of issues with
GifBuilder and friends.

Change-Id: Id22c7636da76778b75f087d20d7b7260cd2637ee
Resolves: #94333
Related: #94057
Releases: master
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/69476


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