Add the horizontal mirror voltages

[DominicOram]

See DiamondLightSource/mx-bluesky#521

Also refactors the tests a bit to make it clearer what is under test

Instructions to reviewer on how to test:

1. Confirm dodal connect i03 passes

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 95.20%. Comparing base (32d252c) to head (ccfcce1).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #834   +/-   ##
=======================================
  Coverage   95.20%   95.20%           
=======================================
  Files         120      120           
  Lines        4982     4985    +3     
=======================================
+ Hits         4743     4746    +3     
  Misses        239      239           

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[rtuck99]

Approved