Add CI for synapse_port_db (#6140)

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.

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.d/6140.misc b/changelog.d/6140.misc
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+Add a CI job to test the `synapse_port_db` script.
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