[tests] For testing, run Postgres from Docker (#177)

Instead of assuming a local installation of Postgres (which is a lot
more intrusive), just assume the existence of Docker, and run Postgres
inside a Docker container. This allows running a clean Docker container
in every test, which will unblock wider testing with Postgres.

This also reduces the dependencies for running the test suite from
requiring Postgres to only requiring Docker, which should be fine to
assume since we already use Docker for a bunch of stuff in LNT.

Finally, this also moves the installation.shtest from being a unit test
to something that we only run in the CI. This was a slow test which
required the network and really didn't belong in a Lit test.

This patch was created with Claude's assistance but I did review
everything, made manual tweaks, etc.