Add some basic tests that the DNS resolver isn't leaking timers

It's a pretty crude test - among other things, it relies on `stop_timer` only setting `@timer` to nil once it's cancelled the timer.

But it's hard to do better - since `@timer` does in fact get nil'd out, we can't poke to see if the timer was cancelled, and since cancellation of periodic timers doesn't deregister the callback, we can't look at `EM.instance_variable_get(:@timers)`.