Check backward compatibility of UFL dualspace

[nbouziani]

This PR is just for checking if the upcoming UFL dual space update breaks the current Firedrake.

UFL PR: firedrakeproject/ufl#25

[nbouziani]

This currently fails because firedrake.Constant inherits ufl.Coefficient and the latter performs some checks on the function space (if it is dual or not) in __new__. This has been fixed in #2294 and is I think more of a design issue at the Firedrake level (we could have firedrake.Constant inheriting ufl.Constant). It is something that is (I think) specific to Firedrake so it won't affect other UFL subpackages that don't have this design.