Wait for all tasks to finish on shutdown

[bachya]

Describe what the PR does:

The Runtime object's shutdown is a bit fire-and-forget: we instruct all tasks to cancel, wait for 1 extra iteration of the event loop, and trust that everything cancels correctly. This works, but it's lucky.

This PR modifies things so that as soon as either runtime task is canceled, Runtime waits for both tasks to actually finish canceling before exiting.

Does this fix a specific issue?

N/A

Checklist:

• Confirm that one or more new tests are written for the new functionality.
• Run tests and ensure everything passes (with 100% test coverage).
• Update README.md with any new documentation.
• Add yourself to AUTHORS.md.

[codecov]

Codecov Report

Merging #259 (e11f018) into dev (4006540) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               dev      #259   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         1014      1018    +4     
=========================================
+ Hits          1014      1018    +4     

Impacted Files	Coverage Δ
ecowitt2mqtt/runtime.py	100.00% <100.00%> (ø)

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