Activating some test after PR inclusion

This test had the following report:
"""
The API call is returning 500 when it should be 404
Issue kytos/flow_manager#131
"""
PR kytos/flow_manager#133 closes this issue.

tests/test_e2e_19_flow_manager.py

@@ -139,11 +139,6 @@ def test_030_install_flow_should_fail(self):
                                  headers={'Content-type': 'application/json'})
         assert response.status_code == 400
 
-    """
-    The Api call is returning 500 when should be 404
-    Issue https://github.com/kytos/flow_manager/issues/131
-    """
-    @pytest.mark.xfail
     def test_035_retrieve_flow_from_non_existent_switch_should_fail(self):
         """Test if the flow retrieving process of an invalid
         switch behaves as expected (404 Error)."""