selftests: forwarding: lib: Move smcrouted helpers here

router_multicast.sh has several helpers for work with smcrouted. Extract
them to lib.sh so that other selftests can use them as well. Convert the
helpers to defer in the process, because that simplifies the interface
quite a bit. Therefore have router_multicast.sh invoke
defer_scopes_cleanup() in its cleanup() function.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/410411c1a81225ce6e44542289b9c3ec21e5786c.1750113335.git.petrm@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>