tests/nixpkgs: move nixpkgs module test to dedicated drv

This should be separate from `test-sources` because we want to re-use a
common instance of nixpkgs throughout those tests.

Also, moved the existing nixpkgs module test from `test-sources`.

This partially reverts commit c4ad4d0b2e.

tests/default.nix

@@ -43,6 +43,7 @@ in
   no-flake = callTest ./no-flake.nix { };
   lib-tests = callTest ./lib-tests.nix { };
   maintainers = callTest ./maintainers.nix { };
+  nixpkgs-module = callTest ./nixpkgs-module.nix { };
   plugins-by-name = callTest ./plugins-by-name.nix { };
   generated = callTest ./generated.nix { };
   package-options = callTest ./package-options.nix { };

tests/main.nix

@@ -8,7 +8,6 @@
   pkgs,
   pkgsUnfree,
   self,
-  system,
 }:
 let
   fetchTests = callTest ./fetch-tests.nix { };
@@ -19,14 +18,12 @@ let
     file: name: module:
     mkTestDerivationFromNixvimModule {
       inherit name;
+      # Use a single common instance of nixpkgs, with allowUnfree
+      # Having a single shared instance should speed up tests a little
       module = {
         _file = file;
         imports = [ module ];
-        _module.args = {
-          # Give tests access to the flake
-          inherit self system;
-          inherit (self) inputs;
-        };
+        nixpkgs.pkgs = pkgsUnfree;
       };
       pkgs = pkgsUnfree;
     };

tests/nixpkgs-module.nix

@@ -1,10 +1,40 @@
 {
+  pkgs,
+  lib,
+  linkFarmFromDrvs,
+  self,
+  stdenv,
+}:
+let
+
+  defaultPkgs = pkgs;
+
+  testModule =
+    name: module:
+    let
+      configuration = lib.nixvim.modules.evalNixvim {
+        modules = lib.toList module ++ [
+          {
+            test = {
+              inherit name;
+              buildNixvim = false;
+              runNvim = false;
+            };
+          }
+        ];
+      };
+    in
+    configuration.config.build.test;
+
+in
+linkFarmFromDrvs "nixpkgs-module-test" [
+
   # TODO: expect not setting `nixpkgs.pkgs` to throw
 
-  overlays =
+  (testModule "nixpkgs-overlays" (
     { pkgs, ... }:
     {
-      test.runNvim = false;
+      nixpkgs.pkgs = defaultPkgs;
 
       nixpkgs.overlays = [
         (final: prev: {
@@ -20,21 +50,15 @@
           '';
         }
       ];
-    };
+    }
+  ))
 
   # Test that overlays from both `nixpkgs.pkgs` _and_ `nixpkgs.overlays` are applied
-  stacked_overlays =
+  (testModule "nixpkgs-stacked-overlay" (
+    { pkgs, ... }:
     {
-      inputs,
-      system,
-      pkgs,
-      ...
-    }:
-    {
-      test.runNvim = false;
-
-      nixpkgs.pkgs = import inputs.nixpkgs {
-        inherit system;
+      nixpkgs.pkgs = import self.inputs.nixpkgs {
+        inherit (stdenv.hostPlatform) system;
         overlays = [
           (final: prev: {
             foobar = "foobar";
@@ -70,5 +94,7 @@
           '';
         }
       ];
-    };
-}
+    }
+  ))
+
+]