postgresql: remove thisAttr argument by calling tests directly

Previously, it was not possible to run tests on an overridden derivation, because
the derivation under test was always pulled from pkgs.

With this change, the following will return the same test:

  postgresql_jit.tests

and

  (postgresql.override { jitSupport = true; }).tests

nixos/tests/postgresql-jit.nix

@@ -1,6 +1,7 @@
 { system ? builtins.currentSystem
 , config ? {}
 , pkgs ? import ../.. { inherit system config; }
+, package ? null
 }:
 
 with import ../lib/testing-python.nix { inherit system pkgs; };
@@ -9,14 +10,17 @@ let
   inherit (pkgs) lib;
   packages = builtins.attrNames (import ../../pkgs/servers/sql/postgresql pkgs);
 
-  mkJitTest = packageName: makeTest {
-    name = "${packageName}";
+  mkJitTestFromName = name:
+    mkJitTest pkgs.${name};
+
+  mkJitTest = package: makeTest {
+    name = package.name;
     meta.maintainers = with lib.maintainers; [ ma27 ];
     nodes.machine = { pkgs, lib, ... }: {
       services.postgresql = {
+        inherit package;
         enable = true;
         enableJIT = true;
-        package = pkgs.${packageName};
         initialScript = pkgs.writeText "init.sql" ''
           create table demo (id int);
           insert into demo (id) select generate_series(1, 5);
@@ -45,4 +49,7 @@ let
     '';
   };
 in
-lib.genAttrs packages mkJitTest
+if package == null then
+  lib.genAttrs packages mkJitTestFromName
+else
+  mkJitTest package