nixos/postgresql: fix enableJIT

Make sure that JIT is actually available when using

    services.postgresql = {
      enable = true;
      enableJIT = true;
      package = pkgs.postgresql_15;
    };

The current behavior is counter-intuitive because the docs state that
`enableJIT = true;` is sufficient even though it wasn't in that case
because the declared package doesn't have the LLVM dependency.

Fixed by using `package.withJIT` if `enableJIT = true;` and
`package.jitSupport` is `false`.

Also updated the postgresql-jit test to test for that case.

nixos/modules/services/databases/postgresql.nix

@@ -7,9 +7,18 @@ let
   cfg = config.services.postgresql;
 
   postgresql =
+    let
+      # ensure that
+      #   services.postgresql = {
+      #     enableJIT = true;
+      #     package = pkgs.postgresql_<major>;
+      #   };
+      # works.
+      base = if cfg.enableJIT && !cfg.package.jitSupport then cfg.package.withJIT else cfg.package;
+    in
     if cfg.extraPlugins == []
-      then cfg.package
-      else cfg.package.withPackages (_: cfg.extraPlugins);
+      then base
+      else base.withPackages (_: cfg.extraPlugins);
 
   toStr = value:
     if true == value then "yes"