test-network: wait for the link is activated

test-network: wait for the link is activated

This fixes the following race:
1. when a dummy interface is created, it is initially down state,
2. hence, wait_operstate() may pass before the link is activated,
3. and the ip command bring up the interface before the activation,
4. and networkd activates, that is, brings down the interface,
5. thus, next wait_operstate() timedout, as it waits for the interface up.

To fix the race, let's wait the link is activated, before enter the loop
of wait_operstate().

Fixes #22267.