ObserverMultiplexer.sending_to_single_observer_does_nothing_if_executor_deleted

Intermittent timeout:

```
[ RUN ] ObserverMultiplexer.sending_to_single_observer_does_nothing_if_executor_deleted


99% tests passed, 1 tests failed out of 326

Total Test time (real) = 724.26 sec

The following tests FAILED:
	143 - mir_unit_tests.ObserverMultiplexer.* (Timeout)
```

https://github.com/canonical/mir/actions/runs/25156539762/job/73739773810?pr=4920