[BUGFIX] Fix namespace in FileMountRepositoryTest

Currently a "Tests\Unit" is missing in between.

Resolves: #46845
Releases: 6.1
Change-Id: I2cb4f7fedf826ff35b66ecd4ea600bc60b41123b
Reviewed-on: https://review.typo3.org/19498
Reviewed-by: Wouter Wolters
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring