traceroute: add tests

[ivy]

Summary

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.

Test Plan

traceroute_test:ipv4_baseport  ->  passed  [7.437s]
traceroute_test:ipv4_basic  ->  passed  [5.120s]
traceroute_test:ipv4_dontroute  ->  passed  [10.141s]
traceroute_test:ipv4_firsthop  ->  passed  [9.327s]
traceroute_test:ipv4_gre  ->  passed  [12.194s]
traceroute_test:ipv4_hugepacket  ->  passed  [9.958s]
traceroute_test:ipv4_icmp  ->  passed  [8.437s]
traceroute_test:ipv4_iptos  ->  passed  [6.374s]
traceroute_test:ipv4_maxhops  ->  passed  [5.461s]
traceroute_test:ipv4_nprobes  ->  passed  [4.723s]
traceroute_test:ipv4_sctp  ->  passed  [10.918s]
traceroute_test:ipv4_srcaddr  ->  passed  [7.297s]
traceroute_test:ipv4_srcinterface  ->  passed  [7.399s]
traceroute_test:ipv4_srcroute  ->  passed  [7.046s]
traceroute_test:ipv4_tcp  ->  passed  [17.785s]
traceroute_test:ipv4_udp  ->  passed  [8.638s]
traceroute_test:ipv4_udplite  ->  passed  [5.959s]
traceroute_test:ipv4_unreachable  ->  passed  [5.099s]

[jlduran]

Moved from https://github.com/freebsd/freebsd-src/pull/1652 per my request, as there is greater visibility here.