Adding drop check to Delay tests

It surfaced in #381 that the Delay tests are not comprehensive and they send messages in bursts. This is my initial attempt at adding rate checks and drop checks to the Delay node specific tests. I would like to do some re-factoring and clean-up, before I commit it, but I thought I'd share it so that others don't pick up the work and so the work doesn't get done twice. Please DON'T merge yet, unless you think it's important to have the tests fixed yesterday.