This adds:
* a test sqlite database
* a configuration file for the sqlite database
* a configuration file for a postgresql database (using the credentials in `.buildkite/docker-compose.pyXX.pgXX.yaml`)
as well as a new script named `.buildkite/scripts/test_synapse_port_db.sh` that:
1. installs Synapse
2. updates the test sqlite database to the latest schema and runs background updates on it
3. creates an empty postgresql database
4. run the `synapse_port_db` script to migrate the test sqlite database to the empty postgresql database (with coverage)
Step `2` is done via a new script located at `scripts-dev/update_database`.
The test sqlite database is extracted from a SyTest run, so that it can be considered as an actual homeserver's database with actual data in it.
