refactor "test" to a test

[frrad]

there was some dangling test code, but no actual test that exercised the Matrix class.

[pchtsp]

hello! and thanks. Could you add it importing unittests like the test_pulp.py file example? Thay way they're all tested together.

[frrad]

Done! It also seemed to be necessary to register the test in run_tests.py. Let me know if that was not the right thing to do.