Bug 1595682: Add 'reftest-wait' class to WPT reftest...

Bug 1595682: Add 'reftest-wait' class to WPT reftest contain-size-select-002.html, to avoid race condition between dynamic change & test snapshot. r=jgraham

Differential Revision: https://phabricator.services.mozilla.com/D52724

--HG--
extra : moz-landing-system : lando