def load_tests(loader, standard_tests, pattern):
  """Prevents test discovery in the mixin modules.

  Mixin test cases can't be run directly. They have to be used in conjunction
  with a test class which performs the setup required by the mixin test cases.
  This does not prevent mixin test case discovery in test classes that inherit
  from mixins.

  Args:
    loader: A unit test loader instance. Unused.
    standard_tests: Already loaded test cases.
    pattern: Test method name pattern. Unused.

  Returns:
    Already loaded standard_tests.
  """
  del loader, pattern  # Unused.
  return standard_tests