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 95.02%. Comparing base (c2096b6) to head (baae65e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #834   +/-   ##
=======================================
  Coverage   95.02%   95.02%           
=======================================
  Files         117      117           
  Lines        4765     4768    +3     
=======================================
+ Hits         4528     4531    +3     
  Misses        237      237           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[rtuck99]

Approved