Bug 1377346 - Fixed assertion to use local variable r=chutten,rpapa

Changed assertion from using harness class ping variable to local variable in test. That way, popping the ping doesn't affect
outcome of test. Also check that ping is not None.

MozReview-Commit-ID: AEDJmhJHUxv

--HG--
extra : rebase_source : 9b4b8ae01a036788b47941b15f5e3b12ae9030cc

toolkit/components/telemetry/tests/marionette/tests/unit/test_ping_server_received_ping.py

@@ -15,6 +15,7 @@ class TestPingServer(TelemetryTestCase):
         data = {'sender': 'John', 'receiver': 'Joe', 'message': 'We did it!'}
         headers = {'Content-type': 'application/json', 'Accept': 'text/plain'}
         json_req = requests.post(self.ping_server_url, data=json.dumps(data), headers=headers)
+        ping = self.wait_for_ping()
+        assert ping is not None
         assert json_req.status_code == 200
-        assert len(self.ping_list) == 1
-        assert data['sender'] == self.ping_list[-1]['sender']
+        assert data['sender'] == ping['sender']