Add a ping test to the mss plugin

[ana-cc]

This would be a test to see whether the advertised MSS is enforced. For all targets that advertised an MSS below 1460, we would check:

-whether they respond to ping
-whether they respond to a ping packet equal in size to the adv MSS + 12 bytes
-whether they respond to a ping packet equal in size to the adv MSS + 18 bytes
-whether they respond to a ping packet equal in size to 1472

[irl]

Is this essentially an implementation of #166?

Probably a question for Gorry.