What is the expected enhancement?

Currently the Python unittests for retworkx lack a cohesive organizational structure. As pointed out in #144 there is a mix of some tests splitting across tests/ and tests/graph for tests using PyDiGraph/PyDAG and PyGraph, while others just put all the tests in the same file under tests/ normally using 2 test classes. We really should make these consistent, so either split all tests into subdirectories by the class being tested or have a single module per function and test both classes in that module with a separate test classes. Which pattern we adopt doesn't matter so much as long as its consistent for all tests.