[TASK] Add PHPStan plugin for PHPUnit

Adding phpstan/phpstan-phpunit makes phpstan
"understand" mocking and a couple of related things.

Loading the extension in phpstan.neon reduces removes
a bunch of PHPStan warnings, makes others more specific,
and adds new warnings for some PHPUnit-specific issues.

> composer require --dev phpstan/phpstan-phpunit:^1.0

Resolves: #96690
Releases: main, 11.5
Change-Id: Ib77f18f9492c3891b46d213b4f3981be1ac21c1a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73221


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