scene_rdl2 Python bindings are untested #328

Currently, there are no published tests for scene_rdl2 Python bindings. These are somewhat incomplete and buggy in places. As part of a larger project, I think it would be worthwhile to expose scene_rdl2 to Python as comprehensively as possible. But before doing that, I think the first step is to build up a test suite so that the existing bindings can be tested, fixes for existing bugs can be verified, new bindings can be validated, and regressions can be caught over time.

I propose using Python's built-in unittest library - it works fine, and it doesn't require any extra dependencies. Tests would be added to tests/mod/python/py_scene_rdl2 alongside the existing tests/lib. The new tests will be integrated into the CTest unit tests as a new test, scenerdl2_py_scene_rdl2_tests, so that these run as part of the standard unit label tests. It would also depend on the scenerdl2_scene_rdl2_tests target, in order to use the DSOs that are generated there for testing (fake / test-only SceneObjects).

The tests themselves would mirror the C++ scene_rdl2 tests, as much as is appropriate in Python. For the most part, the file structure should mirror the C++ tests as well, to make it easy to jump back and forth and compare (e.g. TestAttribute.cc would have a paired test_attribute.py).

I"m happy to take on this work if there's no concerns about the approach.