fibs_test: Run in a jail

These tests assume exclusive use of one or more FIBs and so can't be run
in parallel. Running them in a VNET jail sidesteps this problem.

MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: Stormshield

(cherry picked from commit a079f2b52aae74b5469f18dcbef1aff680b80282)