Updates MVK test to be configured through testmod

[jasonb5]

• Updates MVK SystemTest to be configurable via testmod
• Updates SystemTests documentation
• Refactors testmod
• Adds ability to generate documentation from Config class

Test suite: n/a
Test baseline: n/a
Test namelist changes: n/a
Test status: n/a

Fixes #4592
User interface changes?:
Update gh-pages html (Y/N)?:

[jasonb5]

TODO:

• Write documentation
• Refactor discovery usermods/testmods directory.

[github-actions]

You can preview documentation at https://esmci.github.io/cime/branch/update-mvk-config/html/index.html

[jasonb5]

New relevant doc section https://esmci.github.io/cime/branch/update-mvk-config/html/users_guide/testing.html#system-test-mods

[jasonb5]

@rljacob @jgfouca @jedwards4b @mkstratos
How do we feel about merging this and handling any issues as they come? The default behavior and testmods feature work.
The EAMxx use case should be supported with this example of a testmod, where atmchange can be used to customize the inputs and copied to a unique per-instance file (EAMxx would need an update to use the per-instance files).

from CIME.utils import safe_copy
from CIME.utils import run_cmd

component = "eamxx"
ninst = 30

def generate_namelist(case, component, i, filename):
  run_cmd(f"atmchange initial_conditions::perturbation_random_seed = {i*32}")
  run_cmd(f"atmchange ...")
  ...
  safe_copy("namelist_scream.xml", f"namelist_scream_{i:04}.xml")

[jedwards4b]

Okay with me.

[jgfouca]

Fine with me too. Feel free to merge it whenever you think it's ready.