[Form] Tweak to FileTypeTest to avoid warning

tests/Symfony/Tests/Component/Form/Extension/Core/Type/FileTypeTest.php

@@ -25,7 +25,10 @@ class FileTypeTest extends TypeTestCase
     public static function setUpBeforeClass()
     {
         self::$tmpDir = sys_get_temp_dir().DIRECTORY_SEPARATOR.'symfony-test';
-        mkdir(self::$tmpDir, 0777, true);
+
+        if (!file_exists(self::$tmpDir)) {
+            mkdir(self::$tmpDir, 0777, true);
+        }
     }
 
     protected function setUp()