add test for mixed Coefficient Piola transforms

It turns out that a surprising number of tests still pass if Piola transforms aren't applied to subelements of a MixedElement (mixed Poisson tests, mixed Helmholtz tests, ...).  Although a few of the more complicated tests do fail, here is a more simple test.

Note that rtol can be jacked up if the right solver_parameters are passed in (we're mixed so can't use the usual pre_only/LU)