Current master branch is getting segmentation fault error on PHP 7 tests

See https://travis-ci.org/phpbb/phpbb/jobs/75044512#L627
That segfault was reproduced locally on Ubuntu 15.04 64-bit + PHP 7 installation.
After some investigation, it turned out that the cause is test_relative_format_date() in from_format_test.php test which passes standalone but leads to segfault in the tests package running (if you remove the file, the segfault just disappears).

Further experimenting found that date_default_timezone_set('Europe/Paris') (or similar) solves the issue while date_default_timezone_set('UTC') doesn't.