Need to increase File I/O reader unit test coverage

### Issue Type

Other (please describe below)

### Issue Description

Automated Fortran unit tests for the `pio_reader.F90` implementation of the CCPP File I/O object have been added to CAM-SIMA in PR #403.

However, that PR does not cover all potential use cases, mostly due to the lack of NetCDF data needed to trigger those options in the code. There also appears to be some potential issues in the way the tests are built or configured with regards to reading variables that have 4 or more dimensions (which works just fine in a standard CAM-SIMA build).

Given this, it would be good to eventually find or create NetCDF files that allow the tests to cover the missing use cases, and to make sure the testing system itself allows for all possible use cases to be properly exercised in a Github Actions workflow.

### Will this change answers?

No

### Will you be implementing this yourself?

Any Software Engineer can do this

Start by reviewing the existing tests added in PR #403 and the pio_reader.F90 implementation. Identify missing use cases and investigate how NetCDF fixtures and four-or-more-dimensional variables are handled in the GitHub Actions workflow. Done means the missing cases are exercised with suitable NetCDF data and the tests run successfully in CI.

Written by the indexing model from the issue text.