selftests: net: increase the delay for relative cmsg_time.sh test

Slow machines can delay scheduling of the packets for milliseconds.
Increase the delay to 8ms if KSFT_MACHINE_SLOW. Try to limit the
variability by moving setsockopts earlier (before we read time).

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

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

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20240510005705.43069-2-kuba@kernel.org


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