TrialFunction -> TestFunction
Merge pull request #283 from firedrakeproject/ffc_interface_test_fix

TrialFunction -> TestFunction
swap TrialFunction and TestFunction variables to match naming convention
DMPlex: Marking non-exec-halo facets

Halo facets that only touch exec vertices get marked as non-exec.
DMPlex: Adding non-exec halo facets to plex_reordering and facet_ordering
DMPlex: Adding non-exec halo to dof_classes