[TASK] vfsStream: Use namespaced version from phpunit follow-up

Follow-up for #50496

Change-Id: I7f0f453100d3024492387b241013cc7088089103
Resolves: #50504
Releases: 6.2
Reviewed-on: https://review.typo3.org/22649
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters