selftest: packetdrill: Add test for experimental option.

selftest: packetdrill: Add test for experimental option.

The only difference between non-experimental vs experimental TFO
option handling is SYN+ACK generation.

When tcp_parse_fastopen_option() parses a TFO option, it sets
tcp_fastopen_cookie.exp to false if the option number is 34,
and true if 255.

The value is carried to tcp_options_write() to generate a TFO option
with the same option number.

Other than that, all the TFO handling is the same and the kernel must
generate the same cookie regardless of the option number.

Let's add a test for the handling so that we can consolidate
fastopen/server/ tests and fastopen/server/opt34 tests.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250927213022.1850048-7-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>