[ticket/13122] Make phpbb_wrapper_gmgetdate_test more reliable

If you're unlucky, calling time() or similar twice in a row will
give you different results. Facebook runs these tests thousands of
times a day, so we occasionally get bogus failures.

PHPBB3-13122
Using assertGreaterThanOrEqual would probably be better, because that will give you better additional information than just "false does not match expected true".