selftests: Add use case section to fcnal-test

Add use case section to fcnal-test.

Initial test is VRF based with a bridge and vlans. The commands
stem from bug reports fixed by:

a173f066 ("netfilter: bridge: Don't sabotage nf_hook calls from an l3mdev")
cd642898 ("netfilter: bridge: Don't sabotage nf_hook calls for an l3mdev slave")

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>