selftests: rtnetlink: Add operational state test

selftests: rtnetlink: Add operational state test

Virtual devices (e.g., VXLAN) that do not have a notion of a carrier are
created with an "UNKNOWN" operational state which some users find
confusing [1].

It is possible to set the operational state from user space either
during device creation or afterwards and some applications will start
doing that in order to avoid the above problem.

Add a test for this functionality to ensure it does not regress.

[1] https://lore.kernel.org/netdev/20241119153703.71f97b76@hermes.local/

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250717125151.466882-1-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>