"Risky" tests from phpunit 4.1

There were 3 risky tests:

1) phpbb_cache_null_driver_test::test_purge
This test did not perform any assertions

2) phpbb_cache_null_driver_test::test_destroy
This test did not perform any assertions

3) phpbb_passwords_manager_test::test_combined_hash_data
This test did not perform any assertions