Incorporate more extensive documentation on system testing

For CTSM (the next-generation land model for CESM, evolving from CLM), I put together a guide on system testing to bring some new people up to speed:

https://github.com/ESCOMP/ctsm/wiki/System-Testing-Guide

(Old attachment for this: [System-Testing-Guide.rest.txt](https://github.com/ESMCI/cime/files/1383834/System-Testing-Guide.rest.txt))

@mvertens and I feel that much of this can and should be incorporated into the cime user's guide, e.g. here:

http://esmci.github.io/cime/users_guide/testing.html

But I'm not sure what the best way would be to organize this. Specifically:

1. The existing cime testing documentation follows a rather different organization. I put together the ctsm documentation following the organization that made the most sense to me, but I recognize that this might not make the most sense to others. So I'm not sure how to merge this with the existing documentation.

2. It's confusing that some aspects of testing have completely different workflows for ACME vs. CESM. I'm not sure how to lay out the documentation in the least confusing way. Maybe we should have one documentation page for common elements, one for ACME's testing workflow and one for CESM's testing workflow? I'm not quite sure which pieces of what I wrote would go into the common page and which would go into the cesm-specific page.

@jedwards4b @jgfouca @mvertens @rljacob @bertinia - Feel free to make suggestions in comments to this issue, but it might be most efficient if we spend part of a cime meeting discussing this.