selftests: devlink: add a warning for interfaces coming up

NetworkManager (and other daemons) may bring the interface up
and cause failures in quiescence checks. Print a helpful warning,
and take the interface down again.

I seem to forget about this every time I run these tests on a new VM.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>