Add check-buildbot CI config

Summary:
So we can run buildbot tests in CI

Depends on D7364

Test Plan:
Install dependencies:

cd contrib/teamcity
workon abcbot
pip3 install teamcity-messages deepmerge pytest
pip3 install -r ../buildbot/requirements.txt

Run:

./build-configurations.py check-buildbot

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7365