tests: Add documentation for new testing code

Change-Id: Id62ad8e452e640073079e76c9ce5898c8888edee
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Signed-off-by: Sean Wilson <spwilson2@wisc.edu>
Reviewed-on: https://gem5-review.googlesource.com/4882