Document self.assertEqual(expected, observed) pattern

Document self.assertEqual(expected, observed) pattern

Tests should use the following pattern to help reviewers:

  self.assertEqual(expected, observed)

This change documents it in effective_neutron.rst and corrects some test
modules[1] including nonobvious ones[2] as an example.

[1] neutron.tests.unit.agent.l3.test_.*
[2] neutron.tests.unit.agent.l3.test_router_processing_queue

Change-Id: I6d6363541e3fef6f66e808aab00507825205b209