docstr: Explicit wait until the element's classes contains the expected `classname`.
  
  timeout if not set, equals to the fixture's `wait_timeout`
  shortcut to `WebDriverWait` with customized `contains_class`
  condition.
code: def wait_for_contains_class(self, selector, classname, timeout=None):
      """Explicit wait until the element's classes contains the expected `classname`.
  
      timeout if not set, equals to the fixture's `wait_timeout`
      shortcut to `WebDriverWait` with customized `contains_class`
      condition.
      """
      return self._wait_for(
          method=contains_class(selector, classname),
          timeout=timeout,
          msg=f"classname -> {classname} not found inside element within {timeout or self._wait_timeout}s",
      )
  
imports: import os
  import sys
  import time
  import logging
  import warnings
  import percy
  import requests
  from selenium import webdriver
  from selenium.webdriver.support import expected_conditions
  from selenium.webdriver.common.by import By
  from selenium.webdriver.support.wait import WebDriverWait
  from selenium.webdriver.common.keys import Keys
  from selenium.webdriver.common.action_chains import ActionChains
  from selenium.common.exceptions import WebDriverException, TimeoutException, MoveTargetOutOfBoundsException
  from dash.testing.wait import text_to_equal, style_to_equal, class_to_equal, contains_text, contains_class, until
  from dash.testing.dash_page import DashPageMixin
  from dash.testing.errors import DashAppLoadingError, BrowserError, TestingTimeoutError
  from dash.testing.consts import SELENIUM_GRID_DEFAULT
label_desc: Use image node_id 18 for calling the Browser obj's underlying member method code with example usage: obj.wait_for_contains_class(selector, classname, timeout) and returns: self