Similar to wait_for_element_not_visible() - returns nothing.
            As above, will raise an exception if the element stays visible.
            Returns True if successful. Default timeout = SMALL_TIMEOUT.

def assert_element_not_visible(self, selector, by=By.CSS_SELECTOR,
                                   timeout=settings.SMALL_TIMEOUT):
        """ Similar to wait_for_element_not_visible() - returns nothing.
            As above, will raise an exception if the element stays visible.
            Returns True if successful. Default timeout = SMALL_TIMEOUT. """
        if self.timeout_multiplier and timeout == settings.SMALL_TIMEOUT:
            timeout = self.__get_new_timeout(timeout)
        self.wait_for_element_not_visible(selector, by=by, timeout=timeout)
        return True