tests/libalias: Improve testing
gettimeofday(3) is almost as expensive as the calls to libalias.
So the call frequency for this call is reduced by a factor of 1000 in
order to neglect it's influence.
Using NAT entries became more realistic: A communication of a random
length of up to 150 packets (10% outgoing, 90% incoming) is applied
for each entry.
Precision of the execution time is raised to see the trends better.
Reviewed by: kp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30405