Major update to testing section

[rljacob]

Re-org and add to documentation on the create_test system. Better seperate the xml-specific parts and the python dictionary parts. Add better explanation of testmods.

Test suite: built locally Test baseline: N/A Test namelist changes: N/A Test status: bit for bit

User interface changes?: N

Update gh-pages html (Y/N)?: Y

[rljacob]

Someone from CESM please check the "Test control with XML files " section for accuracy. Everything outside of that and "Test control with python dictionaries" should be applicable to both models.

[github-actions[bot]]

You can preview documentation at https://esmci.github.io/cime/branch/rljacob/docs/update-testing/html/index.html

[rljacob]

Need this to be live for tutorial people so merging.