Fix accidentally broken unit test

[DominicOram]

To test:

1. Confirm tests pass

[codecov]

Codecov Report

Merging #676 (477dd56) into main (779cb1f) will increase coverage by 0.64%.
The diff coverage is 95.23%.

@@            Coverage Diff             @@
##             main     #676      +/-   ##
==========================================
+ Coverage   89.11%   89.75%   +0.64%     
==========================================
  Files          34       34              
  Lines        1488     1494       +6     
==========================================
+ Hits         1326     1341      +15     
+ Misses        162      153       -9     

Impacted Files	Coverage Δ
src/artemis/parameters/beamline_parameters.py	97.87% <90.00%> (+12.50%)	⬆️
...rc/artemis/experiment_plans/fast_grid_scan_plan.py	82.87% <100.00%> (+0.44%)	⬆️
src/artemis/experiment_plans/full_grid_scan.py	70.31% <100.00%> (+0.95%)	⬆️
...rtemis/experiment_plans/oav_grid_detection_plan.py	100.00% <100.00%> (+3.44%)	⬆️
src/artemis/parameters/constants.py	100.00% <100.00%> (ø)

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