Rework test framework

[WadeWaldron]

Description

Reworking the test framework to a) take advantage of the native table creation mechanisms in the table API, and b) isolate some of the functionality into a re-usable "testkit".

Checklist

• Unit tests created/updated for any new code (where applicable).
• Run all tests with ./build.sh validate.
• Update the CHANGELOG.md.
• Update the README.md if necessary.

[confluent-cla-assistant]

🎉 All Contributor License Agreements have been signed. Ready to merge.
_{Please push an empty commit if you would like to re-run the checks to verify CLA status for all contributors.}