Bug 1550141 - disable toolkit/modules/tests/xpcshell/test_DeferredTask.js on osx 10.14. r=Yoric

disable toolkit/modules/tests/xpcshell/test_DeferredTask.js on osx 10.14.

Differential Revision: https://phabricator.services.mozilla.com/D32333

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

toolkit/modules/tests/xpcshell/xpcshell.ini

@@ -14,7 +14,7 @@ skip-if = toolkit == 'android'
 [test_Color.js]
 [test_CreditCard.js]
 [test_DeferredTask.js]
-skip-if = toolkit == 'android'
+skip-if = toolkit == 'android' || (os == 'mac' && os_version=='10.14') # osx: Bug 1550141;
 [test_FileUtils.js]
 skip-if = toolkit == 'android'
 [test_FinderIterator.js]