selftests/net: test ipip packets in gro.sh

Add IPIP test-cases to the GRO selftest.

This selftest already contains IP ID test-cases. They are now
also tested for encapsulated packets.

This commit also fixes ipip packet generation in the test.

Signed-off-by: Richard Gobert <richardbgobert@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250923085908.4687-6-richardbgobert@gmail.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>