Add a goal order parameter to the dependency solver

This parameter would allow unit tests to specify the exact order of goals in the solver's tree.  Without it, changes in goal order heuristics could break the backjumping tests that rely on a specific order.  It would also allow us to simplify those tests by specifying the goal order more directly.  This issue came up in https://github.com/haskell/cabal/pull/3422#issuecomment-219205230.
