Bug 1496704 - temporarily disable failing test test-image-insertion-dialog.js. rs=bustage-fix

mail/test/mozmill/composition/test-image-insertion-dialog.js

@@ -30,6 +30,7 @@ function setupModule(module) {
   kh.installInto(module);
 }
 
+test_image_insertion_dialog_persist.__force_skip__ = true;  // See bug 1496704.
 function test_image_insertion_dialog_persist() {
   let cwc = open_compose_new_mail();