tests: Revise FIB lookups per second benchmarking routines

Fix a bug in the LPM SEQ benchmark (missing break inside a switch block)
by restructuring the test loop, while introducing additional two
synthetic test options:

ANN: scan only the address space announced in current RIB
REP: repeat lookups over several keys in a sliding window scheme

The total of eight combinations of test options are now available
through dedicated sysctl hooks.

Differential Revision: https://reviews.freebsd.org/D30311
Reviewed by: melifaro
MFC after: 3 days