selftest: tcp: Add bind() tests for IPV6_V6ONLY.

bhash2 was not well tested for IPv6-only sockets.

This patch adds test cases where we set IPV6_V6ONLY for per-fixture
bind() calls if variant->ipv6_only[i] is true.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20240326204251.51301-8-kuniyu@amazon.com


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