Fix flaky DatePeriod test

$start and $end use the H:i:s from the current time. If $end happens on
a second boundary, $start + 4 days will include $end, thus performing an
extra iteration. Fix this by setting H:i:s to 00:00:00.

Author: iluuu1994

ext/date/tests/DatePeriod_no_advance_on_valid.phpt

@@ -3,8 +3,8 @@ Date Period iterators do not advance on valid()
 --FILE--
 <?php
 
-$start = DateTime::createFromFormat('Y-m-d', '2022-01-01');
-$end = DateTime::createFromFormat('Y-m-d', '2022-01-04');
+$start = DateTime::createFromFormat('Y-m-d H:i:s', '2022-01-01 00:00:00');
+$end = DateTime::createFromFormat('Y-m-d H:i:s', '2022-01-04 00:00:00');
 $interval = DateInterval::createFromDateString('1 day');
 $period = new DatePeriod($start, $interval, $end);
 $iterator = $period->getIterator();