traceroute: add tests

add some basic tests for traceroute. this covers most of the flags we
can easily test; in some cases we use tcpdump to ensure the correct
packets are actually being sent.

to run the tests, we create three jails: one for the source host, one
for the destination host, and one to route packets betweem them. this
ensures we're actually testing traceroute across a routed network and
not just sending probe packets to a directly connected host.

no tests for traceroute6 are in this commit since the traceroute6 merge
into traceroute is in progress elsewhere.

Reviewed by: des, adrian
Approved by: des (mentor)
Differential Revision: https://reviews.freebsd.org/D49838