Bug 1600208 - mark test_bug756984 as permafail on ubuntu1804 r=jmaher

Changes:

This test is a permafail on ubuntu1804, and should be marked as such as the migration to ubunt1804 is actively underway with target date of end of December 2019.

With a fail-if annotation, the hope is that if this test suddenly begins passing we will be notified of it (as opposed to skipping it outright).

Differential Revision: https://phabricator.services.mozilla.com/D57381

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

layout/generic/test/mochitest.ini

@@ -85,6 +85,7 @@ support-files = bug633762_iframe.html
 [test_bug735641.html]
 [test_bug748961.html]
 [test_bug756984.html]
+fail-if = (os == 'linux' && os_version == '18.04') # Bug 1600208 permafail on ubuntu1804
 [test_bug784410.html]
 [test_bug785324.html]
 [test_bug791616.html]