code: def _get_new_features_name(self):
      return [f"{i}_plus" for i in self.variables_]
  
imports: import numpy
  import pytest
  from sklearn.exceptions import NotFittedError
  from sklearn.impute import SimpleImputer
  from sklearn.pipeline import Pipeline
  from sklearn.preprocessing import PolynomialFeatures
  from feature_engine._base_transformers.mixins import GetFeatureNamesOutMixin
  from feature_engine.dataframe_checks import check_X
label_desc: Use image node_id 4 for calling the MockCreator obj's underlying member method code with example usage: obj._get_new_features_name() and returns: unknown