feature/echo branch 

This is a copy of the echo test which was specified in the bridge reachability testing ticket, and this branch has been cleaned up to remove unrelated commits. It includes some necessary bug fixes for txscapy. I ended up rewriting the test to *not* use the scapy test template because the template was broken in some cases, and attempting to fix all the scapy related tests and fix the template at the same time was obnoxious; it seemed a better idea to have tests which definitely work correctly and then make sure that any changes to the template do not break any of  the tests. 

It's been long enough now that I no longer remember what the problems with the template were, though there is a chance I have records of it in my testing logs.