Add test helper spec for Parasitic Traceroute

[willscott]

In addition to the forward and reverse trace route helpers currently defined, it will be useful to develop a test helper specifically to encode the process of conducting a 'parasitic' trace route on a tcp connection initiated by a client.

This spec addition attempts to formalize the behavior of such a helper.

[willscott]

I've completed a second, golang, version of traceroute-as-a-service that will function as a test helper for this form of test: http://github.com/willscott/traas2

[hellais]

This should probably include also some mentions of the data format. @willscott do you think you can add to this the specs of the traa2 data format?

I think that is a good start for it, once that is done I think this can be merged.