investigate/fix test flake: confirmation popup timeout in functional tests

[redshiftzero]

Description

Test flake while waiting for a confirmation pop up in test_admin_adds_admin_user:

    def until(self, method, message=''):
        """Calls the method provided with the driver as an argument until the \
            return value is not False."""
        screen = None
        stacktrace = None
    
        end_time = time.time() + self._timeout
        while True:
            try:
                value = method(self._driver)
                if value:
                    return value
            except self._ignored_exceptions as exc:
                screen = getattr(exc, 'screen', None)
                stacktrace = getattr(exc, 'stacktrace', None)
            time.sleep(self._poll)
            if time.time() > end_time:
                break
>       raise TimeoutException(message, screen, stacktrace)
E       TimeoutException: Message: Timed out waiting for confirmation popup.

e.g. https://circleci.com/gh/freedomofpress/securedrop/29653

[redshiftzero]

reopening for monitoring because @rmol reported seeing this again