passwords_manager_test::test_unique_id fails from time to time

There was 1 failure:
1) phpbb_passwords_manager_test::test_unique_id
Failed asserting that '337579e221828689' is not equal to string:768555e745534201.
/home/travis/build/phpbb/phpbb/tests/passwords/manager_test.php:276

https://github.com/phpbb/phpbb/pull/2222#issuecomment-39583335

As per bantu:

337579 * 10 ^ 221828689 = INF = 768555 * 10 ^ 745534201
Should be using assertSame instead of using assertEquals everywhere.