Add a comprehensive test suite

[gskinner]

It would be nice to have a unified test suite XD file. This would serve multiple purposes:

1. Facilitate more robust testing when preparing new releases.
2. Demonstrate supported vs unsupported vs broken features in the plugin.
3. Provide a test suite for community members preparing pull requests.

This effort is already well underway, but I thought there should be an issue to track it.

[gskinner]

This is largely complete, but I'm still sorting out a few details, so it will ship with the next release.

[gskinner]

Continuing to update this locally, but didn't ship with v2.0.0