def is_test_directory_name(directory_name):
    """
    Returns whether the given directory name is a name of a test directory.
    
    Parameters
    ----------
    directory_name : `str`
        A directory's name.
    
    Returns
    -------
    is_test_directory_name : `bool`
    """
    if directory_name == 'tests':
        return True
    
    if directory_name.startswith('test_'):
        return True
    
    if directory_name.endswith('_tests'):
        return True
    
    return False