[TASK] Use first-class callables in EXT:extensionmanager unit tests

see https://www.php.net/manual/en/functions.first_class_callable_syntax.php

Resolves: #100315
Releases: main
Change-Id: Ib915e7329d2a803dca241f533e0297c4cd13e204
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78270


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>