[Filesystem] Fix test suite on OSX

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT

Reviewing PR's, I realized the filesystem test suite fails on my setup (OSX, PHP 5.5.13 with posix ext). `posix_getgrgid` returns false, so some tests are failing.
This solves this issue.

Be aware that the patched method has been moved in FilesystemTestCase in recent branches