Port testing to pytest

[davidrudlstorfer]

To simplify the current testing setup and to benefit from additional pytest features the testing suite should be moved to pytest.

[isteinbrecher]

This is for sure a great improvement to the testing of MeshPy! Maybe we can also use this refactoring of the testing to “clean up” the testing structure into more clearly defined subsets of tests. I will think about what could be a good path to go here. However, does not hinder the conversion of the current structure to pytest.