[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
Yep. Just style; I like the asserts to specifically test one thing - one for the ordering, one for the maximum interval.

If I got rid of the abs, the first assert would also be redundant, but I think having that separate makes it clearer.