Minor MQTT updates

[skotl]

Non-breaking change

Proposed change

Minor updates to MQTT, one functional (slightly) and the other two in the DebugHost app:

1. When MQTT cannot connect, no useful message was logged - this has been fixed
2. I forgot how to test MQTT message subscriptions(!) so added a comment on how to do this to the code
3. Refactored the original MQTT exercisor app, which was a bit of a mess - consistent IDs and now all are deleted when the test completes

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the [development checklist][dev-checklist]
• The code compiles without warnings (code quality chek)
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.28% ⚠️

Comparison is base (8382bd0) 81.17% compared to head (743c223) 80.89%.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #906      +/-   ##
==========================================
- Coverage   81.17%   80.89%   -0.28%     
==========================================
  Files         174      174              
  Lines        4350     4361      +11     
  Branches      435      437       +2     
==========================================
- Hits         3531     3528       -3     
- Misses        636      650      +14     
  Partials      183      183              

Flag	Coverage Δ
unittests	80.89% <0.00%> (-0.28%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
...ensions.MqttEntityManager/AssuredMqttConnection.cs	49.09% <0.00%> (-12.28%)	⬇️

... and 1 file with indirect coverage changes

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[helto4real]

Looks good to me