Correct test function for mocking timeout.

[Relm-Arrowny]

Fixes #1060
Unblock #1033

Instructions to reviewer on how to test:

1. run: tox -e tests -- --durations=10 tests/devices/unit_tests/test_apple2_undulator.py
2. Confirm timeout is less than 1s

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 97.69%. Comparing base (4ca1dc0) to head (8e9a52f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1061   +/-   ##
=======================================
  Coverage   97.69%   97.69%           
=======================================
  Files         160      160           
  Lines        6629     6629           
=======================================
  Hits         6476     6476           
  Misses        153      153           

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[DominicOram]

Great, thank you!