[QA] PHP 7 now available and usable in CI

[Legoktm]

Hi!

PHP 7.0 is now available and usable in CI. As a small starter, we have
added a job that runs "composer test" using PHP 7.0 for all repositories
that run some form of MediaWiki or unit tests or composer tests. It's
not enabled by default, so you'll need to run "check experimental" on a
patch to see if it passes (thought I doubt there will be a lot of issues).

You can follow [1] for progress on enabling those jobs by default, and
[2] for the next step of running MediaWiki unit tests under PHP 7.

Thank you to ops and the CI team for all of their help and work on this :)

[1] https://phabricator.wikimedia.org/T144961
[2] https://phabricator.wikimedia.org/T144962

-- Legoktm