Test # of roundtrips

[sirupsen]

It'd be useful to have a call to be able to know how many roundtrips it did during a certain amount of time. For a test, something like this:

assert_toxiproxy_roundtrips(3) do
  3.times { redis_client.get("hello") }
end

This could help minimize roundtrips in hot-paths and asserting they don't creep above some limit.