Added test case to strengthen YAML configuration validation

[caronc]

Description:

Related issue (if applicable): n/a

It was reported that an Apprise URL could not load a single instance in a YAML configuration tied to a tag.

This was just a test case added to re-produce the scenario

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• No lint errors (use flake8)
• 100% test coverage

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8bff103) 99.27% compared to head (3af779c) 99.27%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1043   +/-   ##
=======================================
  Coverage   99.27%   99.27%           
=======================================
  Files         135      135           
  Lines       17600    17600           
  Branches     3592     3592           
=======================================
  Hits        17472    17472           
  Misses        119      119           
  Partials        9        9           

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