test: introduce test-db

[juleswritescode]

This PR introduces a pg_test_utils crate.

It contains a get_new_test_db function that'll create a new database against a connected postgres instance. We can use that database for testing; e.g. insert a table and test whether the autocompletion works correctly. We assert that the postgres instance runs on either localhost or 127.0.0.1. Note: the test databases aren't automatically cleaned up, but can simply be removed with docker.

I stole the uuid approach from here.

Additionally:

• I changed the DB_CONNECTION_STRING env var to DATABASE_URL, which was already set and required for sqlx
• I ensured that all crates use the workspace's sqlx version
• I updated the packages in the root Cargo.toml because I had dependency conflicts between tokio and threadpool

Next up, I'll write some proper tests for the pg_completions crate. :)