Add test of upwind flux construction

Construct a BDM1 flux F such that div(F) == D0 where D0 is an upwind DG0
approximation to div(u*D).
Merge pull request #284 from firedrakeproject/flux_test

Add a test of upwind flux construction.  Construct a BDM1 flux F such
that div(F) == D0 where D0 is an upwind DG0 approximation to div(u*D).