Interior facet mfs/vfs test

Waiting on
https://github.com/firedrakeproject/firedrake/pull/230
https://bitbucket.org/mapdes/ffc/pull-request/23/ignore-restriction-for-constants/diff
https://bitbucket.org/mapdes/ffc/pull-request/24/fix-zero-form-integrals-with-vector-mixed/diff

An exposition of the fix is in the last of these links.
This fixes https://github.com/firedrakeproject/firedrake/issues/165