[Postgres]: Fix test process not starting

[shivaraj-bh]

On the first run, when data directory doesn’t exist, both the init processes (pg1-init and pg2-init) will try to create the data directory and one of them fails, leading to one of the server not starting. To solve this, pg2-init can depend on pg1-init completing.

Also depends_on option added to postgres here, will not be needed because we can do it as demonstrated in example/flake.nix and nix/postgres/postgres_test.nix (check the diff). This keeps the definition of depends_on in one-place.

Resolves #74