Fix flaky timezone test fixture (GH-18624)

As documented in [RunningTests], tests have to be written to be
independent of any php.ini file.

Without specifying the timezone, the var_dump() will make use of the
default timezone that may not be the expected "UTC" timezone.

This renders the test-cases flaky and will make them fail if the
`date.timezone` is different from the string "UTC".

Fix is to interpolate the timezone in the test fixture or to set
`date.timezone` to `UTC`.

[RunningTests]: docs/source/miscellaneous/running-tests.rst
fix-up-of: GH-18624

Author: hakre

ext/date/tests/multiple_calls_date_period_iterator_current.phpt

@@ -2,8 +2,8 @@
 Multiple calls to DatePeriod iterator current() leak objects
 --FILE--
 <?php
-$start = new DateTime('2018-12-31 00:00:00');
-$end   = new DateTime('2019-12-31 00:00:00');
+$start = new DateTime('2018-12-31 00:00:00 UTC');
+$end   = new DateTime('2019-12-31 00:00:00 UTC');
 
 $interval = new DateInterval('P1M');
 $period = new DatePeriod($start, $interval, 1);