selftests: drv-net: wait for carrier

On fast machines the tests run in quick succession so even
when tests clean up after themselves the carrier may need
some time to come back.

Specifically in NIPA when ping.py runs right after netpoll_basic.py
the first ping command fails.

Since the context manager callbacks are now common NetDrvEpEnv
gets an ip link up call as well.

Reviewed-by: Joe Damato <joe@dama.to>
Link: https://patch.msgid.link/20250812142054.750282-1-kuba@kernel.org


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