add --no-subscribe-all option

[tiniuclx]

This pull request adds a pytest command line option called --no-subscribe-all which disables the automatic subscription to all topics. When using this option, tests can be ran against a busy production MQTT broker which would have a lot of traffic that is not interesting to the test itself.

Subscriptions can be made manually in each test using capmqtt.mqtt_client.client.subscribe("topic/goes/here")

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.19%. Comparing base (1acfb43) to head (8ca8fac).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #56      +/-   ##
==========================================
+ Coverage   96.08%   96.19%   +0.10%     
==========================================
  Files           5        5              
  Lines         179      184       +5     
==========================================
+ Hits          172      177       +5     
  Misses          7        7              

Flag	Coverage Δ
unittests	96.19% <100.00%> (+0.10%)	⬆️

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[tiniuclx]

@amotl this is ready for review - I would appreciate a review & merge whenever you have the chance :)