Bug 1572819 - Enable toolkit/mozapps/extensions/test/xpcshell/test_system_update_enterprisepolicy.js for Thunderbird; r=mkaply

Differential Revision: https://phabricator.services.mozilla.com/D42083

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

toolkit/mozapps/extensions/test/xpcshell/xpcshell.ini

@@ -130,7 +130,6 @@ head = head_addons.js head_system_addons.js
 skip-if = true # Failing intermittently due to a race condition in the test, see bug 1348981
 [test_system_update_enterprisepolicy.js]
 head = head_addons.js head_system_addons.js
-skip-if = appname == "thunderbird"
 [test_system_update_fail.js]
 head = head_addons.js head_system_addons.js
 skip-if = true # Bug 1501879