[BUGFIX] Fix namespace in FileMountRepositoryTest

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

Resolves: #46845
Releases: 6.1, 6.0
Change-Id: I2cb4f7fedf826ff35b66ecd4ea600bc60b41123b
Reviewed-on: https://review.typo3.org/24450
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn