Move sub and sub-subtest documentation into their modules

Putting the docs. next to the code solves multiple problems:

*  Un-clutter main documentation
*  Help keep docs current by closer proximity to likely areas of change
*  Provide reporting-accessable human-readable description mapping to test names.
*  Allow for easier verification of general formatting in Travis CI.
I believe booth of these are kind-of essential. I see `Prerequisites` as place for very specific settings as another server running NFS, or similar...