This test was incorrectly exercising a member of the endpoint record returned
by `Manager.get_endpoints()`. In the test environment, the `.listen` Endpoint
is actually a Mock, so calling e.g. `listen()` on `endpoints.listen` returns
another Mock instead of a Deferred. Twisted's `assertNoResult` used to
tolerate this silently, but as of Twisted-19.10 it throws an error, causing
the test to fail.
The fix is to assert that the record has attributes with the right names, but
not assume they behave like normal Endpoints, and not call `.listen()` on
them.
closes#366
6be50a4f0a