Refactor MG Centrality Tests

[nv-rliu]

Addresses #4187

This PR makes improvements to old testing conventions used in cugraph.centrality MG tests

Instead of using nested fixtures. eg. input_expected_output -> input_combo -> fixture_params which can be confusing, the @pytest.mark.parametrize marker is used to iterate through combinations of parameters used for testing. The fixtures are also replaced by functions used to create SG and MG graphs.