Add testing TODOs

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42537 0bd85f0b-c151-43b9-a1e9-27231104551f
suzerain: #2503 use "_xi" suffix instead of "_p"

I will be adding \partial_y derivatives, and having
the _xi suffix in place is good as the changes will
soon require _y.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42538 0bd85f0b-c151-43b9-a1e9-27231104551f