Should have used DRUPAL_ROOT instead of $_SERVER['DOC_ROOT']

Quick patch to the pull request I made. I should have used DRUPAL_ROOT
instead of the php document root variable as the patch didn't work on a
second system otherwise

tests/MimeClass.test

@@ -10,8 +10,8 @@
 //file_mimetype_mapping wasn't getting loaded. 
 //the solution was to include it in the test
 
-$path = "/drupal7/includes/file.mimetypes.inc";
-require_once $_SERVER['DOCUMENT_ROOT'] . $path;
+$path = "/includes/file.mimetypes.inc";
+require_once DRUPAL_ROOT . $path;
 
 class MimeClassTestCase extends DrupalWebTestCase {
   protected $privileged_user;