[ticket/12354] Assert same instead of equal where necessary in passwords tests

In some tests we try to make sure that we do not get the same output in a
limited timeframe. These tests should use assertSame/NotSame instead of
assertEqual/NotEqual.

PHPBB3-12354
Merge remote-tracking branch 'Marc/ticket/12354' into develop-ascraeus

* Marc/ticket/12354:
  [ticket/12354] Assert same instead of equal where necessary in passwords tests