fibs tests: Fix test failures and simplify

The tests previously expected the invoker to pass a list of FIBs to use.
However, they now run in separate VNET jails, so we can simply expand
the FIB array as needed in each test. Modify each test to simply set
net.fibs as needed and grab FIB numbers starting at 1.

A number of tests were also broken by commit 9206c7996198
("usr.bin/netstat: -n should not print symbolic names"), so fix those.

Reviewed by: asomers
Fixes: 9206c7996198 ("usr.bin/netstat: -n should not print symbolic names")
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D48585