test/conn-unreach: Accept -ENOTCONN from shutdown on unconnected socket

When the host has no route to the unreachable address (e.g., only
loopback interface is up), connect never completes, and the subsequent
shutdown(SHUT_RDWR) may return -ENOTCONN from the kernel.

Relax the check to accept both 0 and -ENOTCONN as valid results.

Signed-off-by: KaFai Wan <kafai.wan@linux.dev>