Currently the tests always modifies the DOM via `setTimeout()`.  In practice, however, the DOM and even individual elements might be modified multiple times before the event loop can execute timers.  It seems this might influence how mutation events are delivered to the monitor.  For example, see [comment 52][] in the bug.

Please add some test operations that synchronously add/remove nodes without yielding the event loop.

[comment 52]: https://bugzilla.mozilla.org/show_bug.cgi?id=865750#c52