test: add basic test for flakeModule functionality

tests/flake_parts_test.nix

@@ -0,0 +1,39 @@
+{
+  pkgs,
+  ntlib,
+  ...
+}: {
+  suites."flake-parts" = {
+    pos = __curPos;
+    tests = [
+      {
+        name = "flakeModule";
+        type = "script";
+        script =
+          # sh
+          ''
+            ${ntlib.helpers.scriptHelpers}
+            ${ntlib.helpers.path (with pkgs; [coreutils nix gnused gnugrep jq])}
+            repo_path=${../.}
+
+            dir=$(mktemp -d)
+            trap "rm -rf $dir" EXIT
+            cd $dir
+            cp ${./fixtures/flake_parts}/* .
+            # import from the absolute path above, is easier than trying to figure out the repo path etc.
+            sed -i -e "s|@repo_path@|$repo_path|" flake.nix
+
+            # NOTE: --impure is required since importing modules from absolute paths is not allowed in pure mode
+            nix build --impure .#gitlab-ci:pipeline:default
+            assert "-f result" "should exist"
+            assert_file_contains "result" "finalize_nix_ci"
+            jq '.' result  # check if valid json just to be sure
+
+            nix build --impure .#gitlab-ci:pipeline:default:pretty
+            assert "-f result" "should exist"
+            assert_file_contains "result" "  - finalize_nix_ci"
+          '';
+      }
+    ];
+  };
+}