Add better tests for shapes

[odow]

We could test that the dot product before and after reshaping is the same.

This would catch issues like #3797

[odow]

@blegat: could you sketch what you had in mind here? Adding some default tests for the shapes doesn't help us show that we use the appropriate shape for a constraint? (Which was the issue here.)

[blegat]

I think it can just be something like

function test_shape(primal, dual, shape)
    vec_primal = vectorize(primal, shape)
    vec_dual = vectorize(dual, dual_shape(shape))
    @test primal = reshape(vec_primal, shape)
    @test dual == reshape(vec_dual, dual_shape(shape))
    @test dot(primal, dual) == dot(vec_primal, vec_dual)
end

[odow]

Sure. But this doesn't really help us catch the fact that the solver might return a "different" value to the one we expected (if we didn't think clearly) for the dual. That would just tell us that Symmetric works. But it would't help tell us we needed the adjoint? (Unless we know good values for primal and dual to test?)

[blegat]

You're right, then we can do something like:
I think it can just be something like

function test_shape(primal, dual, con::VectorConstraint)
    vec_primal = vectorize(primal, con.shape)
    vec_dual = vectorize(dual, dual_shape(con.shape))
    @test primal = reshape(vec_primal, con.shape)
    @test dual == reshape(vec_dual, dual_shape(con.shape))
    @test dot(primal, dual) == MOI.Utilities.set_dot(vec_primal, vec_dual, con.set)
end