test: refactor, by factoring 'pkgs' in its own module

We don't need per-file package overlays, since we don't envision having too many of them. Whenever possible, get them from nixpkgs, to keep our tests nimble.

test/flake.nix

@@ -5,24 +5,15 @@
     systems.url = "github:nix-systems/default";
     process-compose-flake.url = "github:Platonic-Systems/process-compose-flake";
     services-flake = { };
-
-    # Deal with broken packages:
-    # grafana is broken on aarch64-darwin: https://github.com/NixOS/nixpkgs/issues/273998
-    nixpkgs-grafana.url = "github:nixos/nixpkgs/b604023e0a5549b65da3040a07d2beb29ac9fc63";
   };
   outputs = inputs:
     inputs.flake-parts.lib.mkFlake { inherit inputs; } {
       systems = import inputs.systems;
       imports = [
         inputs.process-compose-flake.flakeModule
+        ./nix/pkgs.nix
       ];
       perSystem = { self', inputs', pkgs, system, lib, ... }: {
-        _module.args.pkgs = import inputs.nixpkgs {
-          inherit system;
-          # Required for elastic search
-          config.allowUnfree = true;
-          overlays = import ./overlays { inherit system inputs' lib; };
-        };
         process-compose =
           let
             mkPackageFor = mod:

test/nix/pkgs.nix

@@ -0,0 +1,29 @@
+{ inputs, ... }:
+
+{
+  perSystem = { self', inputs', pkgs, system, lib, ... }: {
+    _module.args.pkgs = import inputs.nixpkgs {
+      inherit system;
+
+      # Required for elastic search
+      config.allowUnfree = true;
+
+      overlays = [
+        (self: super: lib.optionalAttrs super.stdenv.isDarwin {
+
+          # grafana is broken on aarch64-darwin, but works on older nixpkgs:
+          # https://github.com/NixOS/nixpkgs/issues/273998
+          grafana = (builtins.getFlake "github:nixos/nixpkgs/b604023e0a5549b65da3040a07d2beb29ac9fc63").legacyPackages.${system}.grafana;
+
+          # Disable tests, because they are failing on darwin:
+          # https://github.com/NixOS/nixpkgs/issues/281214
+          pgadmin4 = super.pgadmin4.overrideAttrs (_: {
+            doInstallCheck =
+              false;
+          });
+
+        })
+      ];
+    };
+  };
+}