`assert_model_roundtrip` should be a `helper`

[WilliamJamieson]

A few packages are importing directly from the asdf_astropy unit tests, the assert_model_roundtrip function, see this example. Things within unit tests are normally considered private; however, this function seems useful for testing converters in other contexts.

I think the assert_..._roundtrip functions should be moved to https://github.com/astropy/asdf-astropy/blob/main/asdf_astropy/testing/helpers.py so that they are properly available for other packages to use.