add some more tests for coverage

[phlptp]

This PR adds some more coverage tests

Also fixes some confusing error situations with config files, that came up while trying some tests for coverage.

[henryiii]

Hmm, something is broken:

[2022-09-29T13:27:00.234Z] ['info'] ->  Issue detecting commit SHA. Please run actions/checkout with fetch-depth > 1 or set to 0
[27](https://github.com/CLIUtils/CLI11/actions/runs/3151673634/jobs/5125925566#step:8:28)
[2022-09-29T13:27:00.236Z] ['info'] Pinging Codecov: https://codecov.io/upload/v4?package=uploader-0.3.2&token=*******&branch=coverage_tests&build=3151673634&build_url=https%3A%2F%2Fgithub.com%2FCLIUtils%2FCLI11%2Factions%2Fruns%2F3151673634&commit=8065aceb23387a087e894e738dc361f92e5f0302&job=Tests&pr=781&service=github-actions&slug=CLIUtils%2FCLI11&name=&tag=&flags=&parent=
[28](https://github.com/CLIUtils/CLI11/actions/runs/3151673634/jobs/5125925566#step:8:29)
[2022-09-29T13:27:00.562Z] ['error'] There was an error running the uploader: Error uploading to [https://codecov.io:](https://codecov.io/) Error: There was an error fetching the storage URL during POST: 404 - {'detail': ErrorDetail(string='Unable to locate build via Github Actions API. Please upload with the Codecov repository upload token to resolve issue.', code='not_found')}

[henryiii]

Trying a couple of things to see if I can fix coverage reporting.

[codecov]

Codecov Report

Base: 99.19% // Head: 99.29% // Increases project coverage by +0.10% 🎉

Coverage data is based on head (931e730) compared to base (dcbcb47).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #781      +/-   ##
==========================================
+ Coverage   99.19%   99.29%   +0.10%     
==========================================
  Files          16       16              
  Lines        3967     3973       +6     
==========================================
+ Hits         3935     3945      +10     
+ Misses         32       28       -4     

Impacted Files	Coverage Δ
include/CLI/impl/App_inl.hpp	99.20% <100.00%> (+<0.01%)	⬆️
include/CLI/ConfigFwd.hpp	98.11% <0.00%> (-1.89%)	⬇️
include/CLI/Option.hpp	100.00% <0.00%> (+0.47%)	⬆️
include/CLI/impl/Option_inl.hpp	99.74% <0.00%> (+1.00%)	⬆️

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[henryiii]

Working. :)