selftest: packetdrill: Refine tcp_fastopen_server_reset-after-disconnect.pkt.

selftest: packetdrill: Refine tcp_fastopen_server_reset-after-disconnect.pkt.

These changes are applied to follow the imported packetdrill tests.

  * Call setsockopt(TCP_FASTOPEN)
  * Remove unnecessary accept() delay
  * Add assertion for TCP states
  * Rename to tcp_fastopen_server_trigger-rst-reconnect.pkt.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250927213022.1850048-12-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>