class_docstr: A unittest class for testing Backdoor Poisoning attack.
code: def poison_func_1(self, x):
      max_val = np.max(self.x_train_mnist)
      return np.expand_dims(
          add_pattern_bd(x.squeeze(3), pixel_value=max_val), axis=3
      )
  
imports: from __future__ import absolute_import, division, print_function, unicode_literals
  import os
  import logging
  import unittest
  import numpy
  from art.attacks.poisoning.backdoor_attack import PoisoningAttackBackdoor
  from art.attacks.poisoning.perturbations import add_pattern_bd, add_single_bd, insert_image
  from art.utils import to_categorical
  from tests.utils import TestBase, master_seed, get_image_classifier_kr
label_desc: Use image node_id 3 for calling the TestBackdoorAttack obj's underlying member method code with example usage: obj.poison_func_1(x) and returns: np