mir_unit_tests.MultiplexingDispatchableTest unstable

E.g. https://github.com/MirServer/mir/runs/6406069741?check_suite_focus=true#step:5:4790:
```
[ RUN ] MultiplexingDispatchableTest.reentrant_dispatchee_is_dispatched_concurrently
/spread/mir/tests/unit-tests/dispatch/test_multiplexing_dispatchable.cpp:231: Failure
Value of: count
Expected: is > 1
Actual: 1 (of type std::atomic)
[ FAILED ] MultiplexingDispatchableTest.reentrant_dispatchee_is_dispatched_concurrently (1001 ms)
```

Start with the failing MultiplexingDispatchableTest.reentrant_dispatchee_is_dispatched_concurrently test in tests/unit-tests/dispatch/test_multiplexing_dispatchable.cpp at line 231, then review the linked CI run. Determine why count remains 1 instead of exceeding 1; done means the test reliably passes without masking the concurrency check.

Written by the indexing model from the issue text.