fix postgres constraints, add postgres testing

This commit fixes the constraint syntax so it is both valid for
sqlite and postgres.

To validate this, I've added a new postgres testing library and a
helper that will spin up local postgres, setup a db and use it in
the constraints tests. This should also help testing db stuff in
the future.

postgres has been added to the nix dev shell and is now required
for running the unit tests.

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

flake.nix

@@ -32,7 +32,7 @@
 
           # When updating go.mod or go.sum, a new sha will need to be calculated,
           # update this if you have a mismatch after doing a change to thos files.
-          vendorHash = "sha256-Qoqu2k4vvnbRFLmT/v8lI+HCEWqJsHFs8uZRfNmwQpo=";
+          vendorHash = "sha256-4VNiHUblvtcl9UetwiL6ZeVYb0h2e9zhYVsirhAkvOg=";
 
           subPackages = ["cmd/headscale"];
 
@@ -102,6 +102,7 @@
           ko
           yq-go
           ripgrep
+          postgresql
 
           # 'dot' is needed for pprof graphs
           # go tool pprof -http=: <source>