[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