bug symfony#16633 [Filesystem] Fixed failing test due to tempdir symlink (toretto460)

This PR was submitted for the master branch but it was merged into the 2.8 branch instead (closes symfony#16633).

Discussion
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[Filesystem] Fixed failing test due to tempdir symlink

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony#16632
| License       | MIT
| Doc PR        |

Fixes the failing test referred in symfony#16632

Commits
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3aee6b9 Fix: Resolve tempdir symlink, not working on OSX

Author: fabpot

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

@@ -1042,8 +1042,8 @@ public function testTempnamOnUnwritableFallsBackToSysTmp()
         $dirname = $scheme.$this->workspace.DIRECTORY_SEPARATOR.'does_not_exist';
 
         $filename = $this->filesystem->tempnam($dirname, 'bar');
-
-        $this->assertStringStartsWith(rtrim($scheme.sys_get_temp_dir(), DIRECTORY_SEPARATOR), $filename);
+        $realTempDir = realpath(sys_get_temp_dir());
+        $this->assertStringStartsWith(rtrim($scheme.$realTempDir, DIRECTORY_SEPARATOR), $filename);
         $this->assertFileExists($filename);
 
         // Tear down