Convert common fixtures to plain functions

[puddly]

The dependency chains created by pytest fixtures unfortunately make selectively applying fixtures (or figuring out how they work at times) difficult. I've converted the following fixtures to normal functions that usually operate on a single Gateway object:

• zha_device_from_file -> zigpy_device_from_device_data and zigpy_device_from_json
• device_joined -> join_zigpy_device
• zigpy_device_mock -> create_mock_zigpy_device

Going forward, I think mock devices created within fixtures should ideally be replaced with real device mocks created from JSON dumps.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.40%. Comparing base (693a881) to head (dcca820).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #221   +/-   ##
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  Coverage   96.40%   96.40%           
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  Files          61       61           
  Lines        9359     9359           
=======================================
  Hits         9023     9023           
  Misses        336      336           

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