[netstack3] Add emulated latency to testutil

- Adds emulated latency in packet deliveries to DummyNetwork
- Changes backing for timers type of DummyEventDispatcher from BTreeMap
to a BinaryHeap, allowing two timers to exist with the same target
Instant.

Change-Id: Ib5e96ea789c71d29466810a0f3ffcae4c68c3102