Describe the bug
Currently pvlib.irradiance.poa_horizontal_ratio is untested according to codecov

To Reproduce
Steps to reproduce the behavior:

1. Go to https://app.codecov.io/gh/pvlib/pvlib-python/blob/main/pvlib/irradiance.py#L259

Expected behavior
100% test coverage

Screenshots

Versions:

• pvlib.__version__: 0.9.5-dirty
• pandas.__version__: ?
• python: ?

Additional context
This causes codecov to fail for any edits to irradiance.py that reduce the number of lines because then code coverage goes down.