Test mw repos have php-composer-validate job

A previous change proposed to add php-composer-validate to all MediaWiki
related extensions: https://gerrit.wikimedia.org/r/#/c/173457/  To avoid
future regression, add an integration tests that ensure the repositories
have php-composer-validate.

The test does NOT check the sub repositories in extensions nor do they
check the mediawiki/services/ hierarchy.

Example usage:

    tox -epy27 -- -s --nologcapture tests/test_zuul_layout.py

Bug: 73444
Change-Id: I6ed918c5cdccade85ed91da4cbdf17de0c29d754