tests/nixpkgs: move nixpkgs module test to dedicated drv

We don't need a full test that actually _builds_ or _runs_ nixvim.
All we need is some assertions on the result of a nixvim configuration.

Author: MattSturgeon

tests/default.nix

@@ -40,6 +40,7 @@ in
   extra-files = callTest ./extra-files.nix { };
   enable-except-in-tests = callTest ./enable-except-in-tests.nix { };
   failing-tests = callTest ./failing-tests.nix { };
+  nixpkgs-module = callTest ./nixpkgs-module.nix { };
   no-flake = callTest ./no-flake.nix { };
   lib-tests = callTest ./lib-tests.nix { };
   maintainers = callTest ./maintainers.nix { };

tests/nixpkgs-module.nix

@@ -0,0 +1,115 @@
+{
+  lib,
+  linkFarmFromDrvs,
+  nixvimConfiguration,
+  runCommandNoCCLocal,
+  self,
+  system,
+}:
+let
+  # Writes a runCommand style derivation based on a nixvim module's output
+  testModule =
+    {
+      name,
+      module,
+      drvArgs,
+      script,
+    }:
+    let
+      configuration = nixvimConfiguration.extendModules {
+        modules = lib.toList module;
+      };
+      finalDrvArgs = {
+        __structuredAttrs = true;
+      } // drvArgs (configuration._module.args // configuration);
+    in
+    runCommandNoCCLocal name finalDrvArgs script;
+
+  # Writes a test derivation that checks specific variables have the expected values
+  testExpectations =
+    {
+      name,
+      module,
+      drvArgs,
+      expectations,
+    }:
+    let
+      checks = lib.concatLines (
+        lib.mapAttrsToList (name: expected: ''
+          ${lib.toShellVar "_expected" expected}
+          if [ "${"$" + name}" != "$_expected" ]; then
+            error+='- Expected ${name} to be "'"$_expected"'" but found "'"${"$" + name}"'"'
+          fi
+        '') expectations
+      );
+    in
+    testModule {
+      inherit name module drvArgs;
+      script = ''
+        error=""
+        ${checks}
+        if [ -n "$error" ]; then
+          echo "${name} failed:"
+          echo "$error"
+          exit 1
+        fi
+        touch $out
+      '';
+    };
+
+  # Further simplifies `testExpectations` when all the expectations relate to attrs in `pkgs`
+  testPkgsExpectations =
+    {
+      name,
+      module,
+      expectations,
+    }:
+    testExpectations {
+      inherit name module expectations;
+      drvArgs = { pkgs, ... }: lib.mapAttrs (name: _: pkgs.${name} or null) expectations;
+    };
+in
+linkFarmFromDrvs "nixpkgs-module-tests" [
+
+  (testPkgsExpectations {
+    name = "overlays-are-applied";
+    module = {
+      nixpkgs.overlays = [
+        (final: prev: {
+          foobar = "foobar";
+        })
+      ];
+    };
+    expectations = {
+      foobar = "foobar";
+    };
+  })
+
+  (testPkgsExpectations {
+    name = "overlays-can-be-stacked";
+    module = {
+      nixpkgs.pkgs = import self.inputs.nixpkgs {
+        inherit system;
+        overlays = [
+          (final: prev: {
+            foobar = "foobar";
+            conflict = "a";
+          })
+        ];
+      };
+      nixpkgs.overlays = [
+        (final: prev: {
+          hello = "world";
+          conflict = "b";
+        })
+      ];
+
+    };
+    expectations = {
+      foobar = "foobar";
+      hello = "world";
+      conflict = "b";
+    };
+  })
+
+]