Refactor tests for minor utility functions

[Eirik Keilegavlen]

Tasks

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1. unit/test_mcolon.py
2. unit/test_permutations.py
3. unit/test_setmembership.py
4. unit/test_graph.py
5. unit/test_half_space.py
6. unit/test_sort_points.py

[Veljko Lipovac]

@keileg Is unit/test_half_space certainly a utility function? Compared to the other files it relies on pp.geometry not pp.utils