Revert "Remove not existing response constant"

This reverts commit b38c36c.

The `Response::HTTP_OK` constant was added in Symfony 2.4. Thus,
removing it (as done in #5771 for the `2.3` branch) must be reverted for
the documentation of the `2.7` branch.

Author: xabbuh

book/testing.rst

@@ -277,7 +277,7 @@ document::
         $this->assertTrue($client->getResponse()->isNotFound());
         // Assert a specific 200 status code
         $this->assertEquals(
-            200,
+            200, // or Symfony\Component\HttpFoundation\Response::HTTP_OK
             $client->getResponse()->getStatusCode()
         );