Interior facet mfs/vfs test

Waiting on
https://bitbucket.org/mapdes/ffc/pull-request/24/fix-zero-form-integrals-with-vector-mixed/diff

An exposition of the fix is in this link.
This is a partial fix for https://github.com/firedrakeproject/firedrake/issues/165 -- the other part needed is fixing the assembly of linear/bilinear forms for VectorFunctionSpaces given in https://bitbucket.org/mapdes/ffc/pull-request/26/fix-assembly-of-linear-bilinear-forms-with/diff (MixedFunctionSpaces happen to be intercepted by the form splitter)