Bad practices in tests

[guimarqu]

In preprocessing tests, one tries to access Coluna's internal Formulations through the JuMP model to test how the preprocessing modifies elements of Formulations. I think it's a bad pratice and we should use JuMP/MOI API instead of doing :

problem.moi_backend.optimizer.inner.re_formulation.master

One could create an algorithm (in tests) that runs the preprocessing and checks the elements. The algorithm will have direct access to all Formulations.

Same for tests of show method.