[TASK] Improve error message in testMethodPrefixChecker

Resolves: #100266
Releases: main
Change-Id: I46ecde712133e57b79989a8a841f05039ddcc9ea
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78225


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>