Use 'Chameleon' skin on en-test.o.o
    
    This is done via pillar so that we can enable the 'Chameleon' skin on
    other wikis with or without making it the default.
    
    I'll drop the 'skin' pillar data again after all wikis are migrated to
    the new design, and also move back $wgDefaultSkin to LocalSettings.php.