Add Buildkite config for Commtest to repo. Docs for the plugin: https://github.com/buildkite-plugins/docker-compose-buildkite-plugin#docker-compose-buildkite-plugin-

Depends on D9512

This config is already applied directly on Buildkite and example tests are passing:
https://buildkite.com/comm/run-commtest/builds/11#018b41f0-e458-4d55-80e2-8dbe15bb1a52