selftests: net: fix timestamp not arriving in cmsg_time.sh

On slow machines the SND timestamp sometimes doesn't arrive before
we quit. The test only waits as long as the packet delay, so it's
easy for a race condition to happen.

Double the wait but do a bit of polling, once the SND timestamp
arrives there's no point to wait any longer.

This fixes the "TXTIME abs" failures on debug kernels, like:

   Case ICMPv4  - TXTIME abs returned '', expected 'OK'

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20240510005705.43069-1-kuba@kernel.org


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