Hack away at runtests

• Do not rely on local copy of dpkt module. Rely on py-dpkt, instead. The reason for this is that this removes the need for introducing localized fixes to dpkt.py, as upstream has changed significantly in the last few years, and now supports both python 2.x and python 3.
• Convert tests to TAP format. This adds improved introspection, as the plain tests have little to no introspection, and kyua lacks python unittest engine support. As such, remove set -ex, as it would impede being able to run this test with TAP cleanly.