[BUGFIX] FollowUp to #56660

Fixes the unit-test according to the changes in review
https://review.typo3.org/#/c/28195/.

Since the BasePath (and therefore vfs://...) cannot be
handover anymore on initialize() it has to be injected
via _set().

Releases: 6.2
Resolves: #56660
Change-Id: I88d03f0305ff08810d6fea82c5c891b1a7a99771
Reviewed-on: https://review.typo3.org/28270
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter