minor #11136 [Filesystem] Fix test suite on OSX (romainneutron)

This PR was merged into the 2.3 branch.

Discussion
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[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

Commits
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e26f08e [Filesystem] Fix test suite on OSX

src/Symfony/Component/Filesystem/Tests/FilesystemTest.php

@@ -981,6 +981,8 @@ class FilesystemTest extends \PHPUnit_Framework_TestCase
         if ($datas = posix_getgrgid($infos['gid'])) {
             return $datas['name'];
         }
+
+        $this->markTestSkipped('Unable to retrieve file group name');
     }
 
     private function markAsSkippedIfSymlinkIsMissing()