[CI] Add Buildkite steps for Commtest

Summary:
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

Test Plan:
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

Reviewers: atul

Reviewed By: atul

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9520