Bug 1561973 - Disabled identity-popup full-screen test on linux. r=nhnt11

Differential Revision: https://phabricator.services.mozilla.com/D36318

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

browser/base/content/test/fullscreen/browser.ini

@@ -2,3 +2,4 @@
 support-files =
   head.js
 [browser_bug1557041.js]
+skip-if = os == 'linux' # Bug 1561973