adding test case for randomized exponential backoff

I noticed there were some plans for composing wait functions in a manner other than max, which sounds interesting. Hopefully this means randomized exponential backoff will be supported out of the box, but in the meantime, here's a helpful test case and a sample custom wait function that implements it. Basic exponential backoff doesn't really alleviate the spiky congestion or rate limited case since it doesn't provide any smoothing function.