Make sure backlight is moved in/out when set

[DominicOram]

Fixes DiamondLightSource/mx-bluesky#198

Instructions to reviewer on how to test:

1. Confirm this would make sure we move the backlight out properly

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 94.54%. Comparing base (da0bb3c) to head (52d2fb0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #767   +/-   ##
=======================================
  Coverage   94.54%   94.54%           
=======================================
  Files         115      115           
  Lines        4617     4622    +5     
=======================================
+ Hits         4365     4370    +5     
  Misses        252      252           

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[olliesilvester]

Maybe should write tests here but the logic is so simple so I doubt I'd have bothered either. How confident are we about the 1 second?

[olliesilvester]

Thanks!