selftests: Add test cases for `ip nexthop flush proto XX`

Add some test cases to allow the fib_nexthops.sh test code
to test the flushing of nexthops based upon the proto passed
in upon creation of the nexthop group.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>