[TASK] Add a function for clearing the rootline caches

The RootlineUtility uses two local caches and one additional cache from
the caching framework. If a unit test uses the RootlineUtility, it needs
a way of purging these caches so it does not change the environment.

Resolves: #58763
Releases: 6.3, 6.2, 6.1
Change-Id: Ia9878b808acfa03f018086829f90b84d4c5e8512
Reviewed-on: https://review.typo3.org/31199
Reviewed-by: Markus Klein
Tested-by: Markus Klein