[TASK] Use explicit types for `willReturnCallback` in EXT:backend tests

This improves both code readability as well as type safety in the tests.

Resolves: #100313
Releases: main
Change-Id: Ib647d5107f48564e847e8c6a9444ae25f83175e7
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78268


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