User's Guide: Testing section is incomplete

[samsrabin]

Specifically, CTSM Build-namelist Tests and Testing PTCLM.

[ekluzek]

I started working on this. The four types of tests that I think should be talked about are:

• run_sys_tests (and CIME create_test)
• python tests
• PF unit tests (also covered in aux_clm
• namelist testing (as pointed out above)

There could also be some talk about testing mksurfdata_esmf, but that is also covered in aux_clm.