[php-src] Issue #12618: Flaky test for IntlDateFormatter [dateformat_format_relative.phpt]

Issue: https://github.com/php/php-src/issues/12618 Author: SakiTakamachi ### Description The test fails at certain times. https://github.com/php/php-src/actions/runs/6767338868/job/18389812623 The following code: ``` <?php $formatter = new IntlDateFormatter( "en_US", IntlDateFormatter::RELATIVE_FULL, IntlDateFormatter::NONE, "America/Los_Angeles", IntlDateFormatter::GREGORIAN, ); echo $formatter->format(new DateTimeImmutable("yesterday")); ``` result: ``` Saturday, November 4, 2023 // now is 2023-11-06 11:08:00 UTC ``` This is due to time zone mismatch. I thought it would be enough to just specify the time zone in DateTimeImmutable, but there are places in the test where it is purposely set to UTC, so I don't know what the purpose of the test is. https://github.com/php/php-src/blob/4022e287b20c310761d15efb64fb14aa080b18ed/ext/intl/tests/dateformat_format_relative.phpt#L53 Is it okay to just set the time zones? Regards. ### PHP Version PHP8.1 ~ master ### Operating System _No response_