selftests: net: explicitly wait for listener ready

The UDP GRO forwarding test still hard-code an arbitrary pause
to wait for the UDP listener becoming ready in background.

That causes sporadic failures depending on the host load.

Replace the sleep with the existing helper waiting for the desired
port being exposed.

Fixes: a062260a

 ("selftests: net: add UDP GRO forwarding self-tests")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/4d58900fb09cef42749cfcf2ad7f4b91a97d225c.1706131762.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>