Fix flakey undulator dcm test

[oliwenmandiamond]

Fixes #2141

Have verified that it is no longer flakey using the event_loop_fuzzing fixture and ran 100 times and all passed as shown below

Instructions to reviewer on how to test:

1. Check new test makes sense

Checks for reviewer

• Would the PR title make sense to a scientist on a set of release notes
• If a new device has been added does it follow the standards
• If changing the API for a pre-existing device, ensure that any beamlines using this device have updated their Bluesky plans accordingly
• Have the connection tests for the relevant beamline(s) been run via dodal connect ${BEAMLINE}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.16%. Comparing base (6b424b6) to head (8c6f719).

Additional details and impacted files

@@                     Coverage Diff                     @@
##           update_to_ophyd_async_0_20    #2142   +/-   ##
===========================================================
  Coverage                       99.16%   99.16%           
===========================================================
  Files                             354      354           
  Lines                           13841    13841           
===========================================================
  Hits                            13725    13725           
  Misses                            116      116           

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