Document what we want covered in system tests

It's hard to rework the system test list without knowing the testing requirements that motivated the addition of various tests. One way to document these requirements is in comments for individual tests, but I'm thinking that, before the next overhaul of the test suite, it could be useful if we write a document listing what we want to test. I'm imagining a table with columns for:

- Aspect we want to test (this could be a compset, a grid, a testmod, etc.)

- What this should be combined with (e.g., we may want a given testmod to be tested with an ERP test with threading)

- What this should *not* be combined with (this will often be N/A, but it could help to list some incompatible options for the sake of guiding the combination of various requirements into fewer tests)