Initial test setup

Making it so that we can test a given form; we find out the highest
order term and make an LHS to handle it. Also, can increase the order
of every term, too (just in case we want something like that).
Only runs script when appropriate
Allows simple expressions now

Was too overzealous with error checking; now we don’t err if there is
an order already seen (as maybe this is from the initial integration
chain).
Testing structure complete

We can now do bunches of tests that have a total given `order’
(counting y’’ + y’ as 5, as there are five symbols between y and
primes, and all other 5’s, etc).

Need to add constants and coefficients.