Use default solver for testing models

Prior to c1e4387 (OCamlPro#829) we did not have
model generation in the default solver, and model generation required to
use the Tableaux solver, so we used the Tableaux solver when testing
models.

Since the default solver now supports model generation, we should use it
to test model generation instead, since the models generated by both
solvers can be different and most users will use the default solver
rather than the Tableaux solver.

Unfortunately, this causes some tests using `get-assignment` to fail
(see OCamlPro#937).