selftests/net: add flush id selftests

Added flush id selftests to test different cases where DF flag is set or
unset and id value changes in the following packets. All cases where the
packets should coalesce or should not coalesce are tested.

Signed-off-by: Richard Gobert <richardbgobert@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20240509190819.2985-4-richardbgobert@gmail.com


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