[TASK] vfsStream: Use namespaced version from phpunit

Newest phpunit introduced a namespaced version of
vfsStream together with the fix for #50230.
Although a migration-layer with the old classnames
was added, the namespaced classes should now
directly be used.

Change-Id: I1832f5690b9ea2b3f9fea5b7a1c2b39f8637f761
Resolves: #50496
Releases: 6.2
Reviewed-on: https://review.typo3.org/22634
Reviewed-by: Oliver Klee
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters