def METHOD_NAME(package_dir):
    """
    Discover and return a sorted list of the names of unit-test modules.
    """
    def is_unittest_module(path):
        file_name = os.path.basename(path)
        return file_name.startswith(UNITTEST_FILE_PREFIX)
    names = get_module_names(package_dir=package_dir, should_include=is_unittest_module)
    # This is a sanity check to ensure that the unit-test discovery
    # methods are working.
    if len(names) < 1:
        raise Exception("No unit-test modules found--\n  in %s" % package_dir)
    return names