Use local broker instead of test.mosquitto.org for tests

[empicano]

There seems to be a race condition in the test_client_unsubscribe test when it runs in the test matrix. This surfaced with the switch to poetry, maybe I messed up something during the switch as well.

When I run the workflows one after the other everything is fine, but when they run at the same time they often fail.

Maybe this could be mitigated by spinning up our own mosquitto broker via Docker for each test run instead of using the public "test.mosquitto.org" one.