Project-wide integration tests

[tim-evans]

We have quite robust unit testing in atjson, but we have encountered quite a few cases where we want to establish systemic properties of atjson as a set of tools and have found difficulty in doing that in unit testing. A good example of us testing these properties is us testing that our commonmark rendering and parsing is compliant to the commonmark spec.

We'd also like to test additional properties of atjson in a full-mesh approach where we can test rigorousness of source libraries. This may be best to do property-based testing as suggested by @colinarobinson to make the testing sufficiently generic while having full coverage.