Bug 1713043 - Stop browser_mailing_lists.js failing on Mac. rs=bustage-fix

I don't know why the end button doesn't work in this situation but nobody seems to be complaining about it so I'm just working around it.

--HG--
extra : rebase_source : 76642d14788bcac7931d15bb1fa328f709e3dfea
extra : amend_source : c02f52b7c4cc6c55d9d2ee9f10d97561759929e0

mail/components/addrbook/test/browser/new/browser_mailing_lists.js

@@ -252,7 +252,7 @@ add_task(async () => {
     // Modify the list's name, nick name, and description fields.
     let modifyField = id => {
       id.focus();
-      EventUtils.sendKey("END", mlWindow);
+      EventUtils.sendKey("DOWN", mlWindow);
       EventUtils.sendString(inputs.modification, mlWindow);
     };
     modifyField(listName);