PeriodicCallback._run was never awaited

[fjetter]

In a couple of different test failures we're seeing RuntimeWarnings about the tornado 6.2 PC

e.g. https://github.com/dask/distributed/actions/runs/3827226701/jobs/6511631823

FAILED distributed/tests/test_actor.py::test_client_actions[True] - pytest.PytestUnraisableExceptionWarning: Exception ignored in: <coroutine object PeriodicCallback._run at 0x7f65440a3790>

Traceback (most recent call last):
  File "/usr/share/miniconda3/envs/dask-distributed/lib/python3.11/warnings.py", line 537, in _warn_unawaited_coroutine
    warn(msg, category=RuntimeWarning, stacklevel=2, source=coro)
RuntimeWarning: coroutine 'PeriodicCallback._run' was never awaited