Consistent test results.

Add leeway in timing tests for early dispatch. This is happening on
occasions but I’ve not seen it happen by more than a millisecond. I’ve
added a timing test to evaluate how often early dispatch happens but I
can’t seem to reproduce the issue now.

10000 tests with a target delay of 0.1 - Simulator
Minimum wait: 0.10003799200058 Maximum wait: 0.110581994056702, Average
wait: 0.104369499695301 Early calls: 0