Ensure non-mocked message publishing during tests fails

In working on #5630 I found that what happens when you don't
properly mock out message publish attempts in tests isn't great.
In CI, the test will hang for several minutes, then eventually
proceed. Obviously we don't want to slow the tests down, and
there is a 30 minute time limit on the test, so if you do this
more than twice or so, the tests will time out. It's not very
clear why the tests are timing out and failing - you have to
guess that it's message publishing causing the problem.

In bcd, the test doesn't hang, it just silently proceeds (I guess
the message gets published to the rabbitmq instance in the bcd
environment, I haven't checked). So you will not notice that you
got it wrong until you submit a PR.

Neither of those behaviors is great, so instead, let's mock out
the 'real' message publishing function in fedora_messaging in
a fixture that's automatically used by every test. We replace
it with a mock that raises an exception with a hopefully-
useful message. We have to mock _twisted_publish, not publish,
because mock_sends also mocks _twisted_publish; if we mocked
publish, we would break mock_sends. This way mock_sends gets
to override us and work OK.

Unfortunately this usually causes a database rollback error so
you have to scroll back through some confusing errors to find
the 'real' problem, but it's still an improvement, I think.

Fixes: #5633

Signed-off-by: Adam Williamson <awilliam@redhat.com>

bodhi-server/tests/conftest.py

@@ -33,6 +33,21 @@ def mock_settings_env_vars():
         yield
 
 
+@pytest.fixture(autouse=True)
+def always_fail_publish():
+    """
+    Make sure any attempt to actually publish a message during a test
+    fails, by mocking out the 'real' message publishing function in
+    fedora_messaging. If you hit this exception, it means you need to
+    use mock_sends, or (if for some reason mock_sends isn't a good
+    fit), mock out notifications.publish or
+    notifications._publish_with_retry.
+    """
+    with mock.patch("fedora_messaging.api._twisted_publish",
+                    side_effect=ValueError("Non-mocked message publish attempted!")):
+        yield
+
+
 @pytest.fixture(scope="session")
 def critpath_json_config(request):
     """