selftests: net: give up on the cmsg_time accuracy on slow machines

selftests: net: give up on the cmsg_time accuracy on slow machines

Commit b9d5f5711dd8 ("selftests: net: increase the delay for relative
cmsg_time.sh test") widened the accepted value range 8x but we still
see flakes (at a rate of around 7%).

Return XFAIL for the most timing sensitive test on slow machines.

Before:

  # ./cmsg_time.sh
    Case UDPv4  - TXTIME rel returned '8074us - 7397us < 4000', expected 'OK'
  FAIL - 1/36 cases failed

After:

  # ./cmsg_time.sh
    Case UDPv4  - TXTIME rel returned '1123us - 941us < 500', expected 'OK' (XFAIL)
    Case UDPv6  - TXTIME rel returned '1227us - 776us < 500', expected 'OK' (XFAIL)
  OK

Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250116020105.931338-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>