Fix test modifying real device registry

[TheJulianJES]

Proposed change

This moves the real_device fixtures to conftest and fixes an issue where the test_local_data_cluster test was modifying the actual DeviceRegistry during tests. That should not happen.

Additional information

Noticed whilst working on #3702

Checklist

• The changes are tested and work correctly
• pre-commit checks pass / the code has been formatted using Black
• Tests have been added to verify that the new code works

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.86%. Comparing base (d80730f) to head (d61391f).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #3703   +/-   ##
=======================================
  Coverage   89.86%   89.86%           
=======================================
  Files         322      322           
  Lines       10387    10387           
=======================================
  Hits         9334     9334           
  Misses       1053     1053           

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