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.