Add meaningful test for linear_map

[schillic]

#1035 fixed a bug where the array of constraints contained undefs. The unit test only checks if the method does not crash. To avoid such problems in the future, we should at least test that each constraint is indeed a constraint.

[mforets]

One could make a simple but meaningful test in such situation by checking for example that the image of an element is in the linear map:

M = N[2 3; 1 2]   # invertible matrix
MP = linear_map(N[2 3; 1 2], P)  # check concrete linear map
@test M * an_element(P) ∈ MP   # check that the image of an element is in the linear map  

[schillic]

These tests have been added starting in #1080.

LazySets.jl/test/unit_Polytope.jl

Line 120 in e4654d7

@test all([M * p ∈ Li for Li in [L1, L2, L3, L4, L5, L6, L7]])