Intermittent eiger stale param unit test failure

[dperl-dls]

I have seen this error pop up occasionally in our CI testing:

=========================== short test summary info ============================
FAILED src/artemis/devices/unit_tests/test_eiger.py::test_given_stale_parameters_goes_high_before_callbacks_then_stale_parameters_waited_on - assert not True
 +  where True = <bound method Thread.is_alive of <Thread(Thread-459 (wait_on_staging), started daemon 140251838105152)>>()
 +    where <bound method Thread.is_alive of <Thread(Thread-459 (wait_on_staging), started daemon 140251838105152)>> = <Thread(Thread-459 (wait_on_staging), started daemon 140251838105152)>.is_alive
===== 1 failed, 196 passed, 1 skipped, 27 deselected, 4 warnings in 12.14s =====

e.g. https://github.com/DiamondLightSource/python-artemis/actions/runs/3998478557/attempts/1

[dperl-dls]

this was fixed in #334