Add missing artifact

Author: mgorny

tests/artifacts/test-plugin-package/test_plugin_package.py

@@ -0,0 +1,8 @@
+class TestPlugin:
+    namespace = "installable_plugin"
+
+    def get_all_configs(self) -> list:
+        raise NotImplementedError
+
+    def get_supported_configs(self) -> list:
+        raise NotImplementedError