[TASK] Increase code coverage of FrontendConfigurationManager

Releases: 6.1
Resolves: #44656
Change-Id: If39fb33a3a3d4cffca3f74f5f15d4a44ebf0e152
Reviewed-on: https://review.typo3.org/17593
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring