Fix FileResource test

I had the following test failing on OS X before:

There was 1 failure:

1) Symfony\Component\Config\Tests\Resource\FileResourceTest::testSerializeUnserialize
Failed asserting that two strings are identical.
merged branch adrienbrault/patch-1 (PR #9016)

This PR was submitted for the master branch but it was merged into the 2.2 branch instead (closes #9016).

Discussion
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[Config] Fix FileResource test

I had the following test failing on OS X before:

```
There was 1 failure:

1) Symfony\Component\Config\Tests\Resource\FileResourceTest::testSerializeUnserialize
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-/var/folders/ph/1jns4kxj653gqg7try5m2k780000gn/T/tmp.xml
+/private/var/folders/ph/1jns4kxj653gqg7try5m2k780000gn/T/tmp.xml

symfony/src/Symfony/Component/Config/Tests/Resource/FileResourceTest.php:56
```

Commits
-------

76a5b50 [Config] Fix FileResource test