selftests: forwarding: Add test cases for unresolved multicast routes

Add IPv4 and IPv6 test cases for unresolved multicast routes, testing
that queued packets are forwarded after installing a matching (S, G)
route.

The test cases can be used to reproduce the bugs fixed in "ipmr: Always
call ip{,6}_mr_forward() from RCU read-side critical section".

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>