Review test with empty polytope

[mforets]

See this line.

Maybe change the current one for this triangle:

julia> A = [N(0) N(1); N(1) N(0); N(-1) N(-1)];
julia> b = N[0.25, 0.25, 0];
julia> P = HPolytope(A, b);

julia> vertices_list(P)
3-element Array{Array{Float64,1},1}:
 [0.25, 0.25]
 [-0.25, 0.25]
 [0.25, -0.25]