Improve portability of Python tests test_allInfo and test_modifyMesh

The following system tests fail on my Mac. A quick look suggests they're probably all due to my machine's name not being in CIME's `config_machines.xml` file. We should think about how we can rewrite them to be machine-independent. This might require the use of a container.

- `test_allInfo`
- `test_modifyMesh`

In addition, it'd be nice avoid calling out to the `nco` utilities. This complicates testing, especially on systems without it installed. The issue is here:
https://github.com/ESCOMP/CTSM/blob/bfc5c3179771a4fabf4ac48975189be80a5eb908/python/ctsm/test/test_sys_mesh_modifier.py#L83-L94

This should be rewritten to pure Python.