[feature/passwords] Move check for 8-bit characters to bcrypt driver

PHPBB3-11610
[feature/passwords] Add tests for unique_id() method in helper

PHPBB3-11610
[feature/passwords] Increase test coverage to 35 out ouf 36 methods

Only one small code part in the salted md5 driver can't be tested right now.
Passwords helper and passwords driver helper are now fully covered by tests.

PHPBB3-11610