tests: fixes & refactoring for nixpkgs-module

modules/top-level/test.nix

@@ -169,6 +169,22 @@ in
       '';
       internal = true;
     };
+
+    # Primarily intended for internal use, allowing us to test `pkgs` while using a `runCommand` from a different nixpkgs instance.
+    runCommand = lib.mkOption {
+      type =
+        with lib.types;
+        addCheck unspecified builtins.isFunction
+        // {
+          description = "runCommand function";
+        };
+      description = ''
+        `runCommand` function used to construct `build.test`.
+      '';
+      defaultText = lib.literalExpression "pkgs.runCommandLocal";
+      default = pkgs.runCommandLocal;
+      internal = true;
+    };
   };
 
   options.build = {
@@ -225,20 +241,20 @@ in
           };
           any = {
             predicate = v: builtins.any (lib.hasInfix v);
-            message = v: "Expected ${lib.toJSON v} infix to be present.";
+            message = v: "Expected ${builtins.toJSON v} infix to be present.";
             valueType = lib.types.str;
           };
           anyExact = {
             predicate = builtins.elem;
-            message = v: "Expected ${lib.toJSON v} to be present.";
+            message = v: "Expected ${builtins.toJSON v} to be present.";
             valueType = lib.types.str;
           };
         };
       };
 
       build.test =
         assert lib.assertMsg (cfg.runNvim -> cfg.buildNixvim) "`test.runNvim` requires `test.buildNixvim`.";
-        pkgs.runCommandLocal cfg.name
+        cfg.runCommand cfg.name
           {
             nativeBuildInputs = lib.optionals cfg.buildNixvim [
               config.build.packageUnchecked

tests/nixpkgs-module.nix

@@ -4,27 +4,34 @@
   linkFarmFromDrvs,
   self,
   stdenv,
+  runCommandLocal,
 }:
 let
 
   defaultPkgs = pkgs;
 
-  testModule =
+  # Only imports the bare minimum modules, to ensure we are not accidentally evaluating `pkgs.*`
+  evalModule =
     name: module:
-    let
-      configuration = lib.nixvim.modules.evalNixvim {
-        modules = lib.toList module ++ [
-          {
-            test = {
-              inherit name;
-              buildNixvim = false;
-              runNvim = false;
-            };
-          }
-        ];
-      };
-    in
-    configuration.config.build.test;
+    lib.evalModules {
+      modules = lib.toList module ++ [
+        {
+          _module.check = false;
+          flake = self;
+          test = {
+            inherit name;
+            buildNixvim = false;
+            runNvim = false;
+            runCommand = runCommandLocal;
+          };
+        }
+        ../modules/misc
+        ../modules/top-level/test.nix
+        ../modules/top-level/nixpkgs.nix
+      ];
+    };
+
+  testModule = name: module: (evalModule name module).config.build.test;
 
 in
 linkFarmFromDrvs "nixpkgs-module-test" [