CI speedups

[zerolab]

This PR does two things:

1. splits test jobs into sqlite and postgres ones, with the latter being the only one getting the postgres service. This is probably by far the biggest improvement (from 10m to 3)
2. Runs the postgres tests with fsync=off and using memory. Refs:
   • https://pythonspeed.com/articles/faster-db-tests/
   • https://stackoverflow.com/questions/75726151/is-it-possible-to-use-tmpfs-on-a-github-workflow-service
   • https://github.com/bitnami/containers/blob/main/bitnami/postgresql/14/debian-11/rootfs/opt/bitnami/scripts/postgresql-env.sh