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

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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

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Additional details and impacted files

```diff @@ 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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