selftests: Add torture tests to nexthop tests

Add Nik's torture tests as a new set to stress the replace and cleanup
paths.

Torture test created by Nikolay Aleksandrov and then I adapted to
selftest and added IPv6 version.

Signed-off-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>