selftests/bpf: Relax TCPOPT_WINDOW validation in test_tcp_custom_syncookie.c.

The custom syncookie test expects TCPOPT_WINDOW to be 7 based on the
kernel’s behaviour at the time, but the upcoming series [0] will bump
it to 10.

Let's relax the test to allow any valid TCPOPT_WINDOW value in the
range 1–14.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/netdev/20250513193919.1089692-1-edumazet@google.com/ #[0]
Link: https://patch.msgid.link/20250514214021.85187-1-kuniyu@amazon.com