It looks like CI runs phpunit --suite skins however the skins suite does not look for tests in a tests folder. It probably should.
This was discovered when debugging T170624. MinervaNeue browser tests fail for RelatedArticles but not for MinervaNeue skin.
It seems like the mw-testskin only runs on testsuite skins but it should also run on testsuite extensions
This blocks merges on:
= Replication steps
 Install MinervaNeue and MobileFrontend make sure MinervaNeue is the only installed skin (Disable the Vector skin locally. )
 Run > php tests/phpunit/phpunit.php --testsuite extensions
 Wait because it's slow...