Add tests for the new module system improvements.

lib/tests/modules/custom-arg-define-enable.nix

@@ -0,0 +1,8 @@
+{ lib, custom, ... }:
+
+{
+  config = {
+    _module.args.custom = true;
+    enable = custom;
+  };
+}

lib/tests/modules/define-if-loaOfSub-foo-enable.nix

@@ -0,0 +1,5 @@
+{ config, lib, ... }:
+
+lib.mkIf config.enable {
+  loaOfSub.foo.enable = true;
+}

lib/tests/modules/define-loaOfSub-foo-enable-if.nix

@@ -0,0 +1,5 @@
+{ config, lib, ... }:
+
+{
+  loaOfSub.foo.enable = lib.mkIf config.enable true;
+}

lib/tests/modules/define-loaOfSub-foo-if-enable.nix

@@ -0,0 +1,7 @@
+{ config, lib, ... }:
+
+{
+  loaOfSub.foo = lib.mkIf config.enable {
+    enable = true;
+  };
+}

lib/tests/modules/define-loaOfSub-if-foo-enable.nix

@@ -0,0 +1,7 @@
+{ config, lib, ... }:
+
+{
+  loaOfSub = lib.mkIf config.enable {
+    foo.enable = true;
+  };
+}

lib/tests/modules/define-module-check.nix

@@ -0,0 +1,3 @@
+{
+  _module.check = false;
+}