authentication_policy: fix UnicodeDecodeError with invalid password messages

When new password does not match policy, in reset password and change password dialogs, we used u' '.join([str(message) ...]) to join all translated messages in a string, but this construct will decode the str(message) to unicode using ascii, so it will fail when these messages contain some multi bytes characters.

Extend test coverage to check that these dialogs uses translations and use non ascii messages in the tests, to make sure we don't have regressions with this issue.