selftests: net: py: avoid all ports < 10k

When picking TCP ports to use, avoid all below 10k.
This should lower the chance of collision or running
afoul whatever random policies may be on the host.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20240429144426.743476-5-kuba@kernel.org


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