[TASK] Use the namespaced vfsStream

The current version of vfsStream (as shipped with the PHPUnit extension)
now uses the namespace \org\bovigo\vfs\vfsStream. We should use it, too.

Change-Id: Ie47782198101af8aab20301c17181542d54098fd
Resolves: #58244
Releases: 6.2
Reviewed-on: https://review.typo3.org/29687
Reviewed-by: Nico de Haen
Tested-by: Nico de Haen