Wait for eiger to disarm before stopping it

[DominicOram]

Fixes #935

Instructions to reviewer on how to test:

1. Confirm new test 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 96.05%. Comparing base (b4c87ae) to head (f36b6b0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #936   +/-   ##
=======================================
  Coverage   96.04%   96.05%           
=======================================
  Files         136      136           
  Lines        5639     5646    +7     
=======================================
+ Hits         5416     5423    +7     
  Misses        223      223           

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