Bug 1601460 - mark browser_fullscreen-newtab.js as expected failure on ubuntu1804 r=jmaher

Changes:

With lack of movement on the bug, mark this test as an expected failure for ubuntu1804 as this bug fails at permafail frequency.

Differential Revision: https://phabricator.services.mozilla.com/D59557

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

dom/html/test/browser.ini

@@ -29,6 +29,7 @@ tags = fullscreen
 tags = fullscreen
 support-files = file_fullscreen-newtab.html
 skip-if = os == 'mac' # bug 1494843
+ail-if = os == 'linux' && os_version == '18.04'  # Bug 1601460
 [browser_submission_flush.js]
 [browser_refresh_after_document_write.js]
 support-files =