Add round-trip and save format tests

[dsmiller95]

This PR adds tests which:

• Assert basic round-trip serialize and deserialize functionality.
• Assert json save format, ideally to make the specific json format more clear. and call out any changes clearly
• Unique round-trip and json save format tests which run when Newtonsoft.Json-for-Unity.Converters package is present

The project used to develop these changes is here: https://github.com/dsmiller95/SaveAsyncEditingProject . Most tests will run in a fresh project, but to see the json-for-unity tests, the development project must also install the json-for-unity (and leave it at default settings).

These tests uncovered one potential issue, documented in #2

[nickpettit]

@dsmiller95 I completely missed this PR! Sorry for the slow response, I'll take a look soon and write more. Thanks so much for contributing!