[BUGFIX] Tests: Namespaced usage of vfsStream

vfsStream was namespaced a while ago. While ext:phpunit still has
a Migration to map non namespaced classes to the namespaced ones,
they should not be used any longer.

Change-Id: I44dcda6707cf6410125f01552c995a27113b3cc2
Resolves: #57872
Related: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/29440
Reviewed-by: Oliver Klee
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring