test_drain_is_called is flaky.

```
 thread 'tests::test_drain_is_called' (22386080) panicked at quickwit-actors/src/tests.rs:623:5:
    assertion `left == right` failed
      left: ProcessAndDrainCounts { process_calls_count: 3, drain_calls_count: 0 }
     right: ProcessAndDrainCounts { process_calls_count: 3, drain_calls_count: 1 }
    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```