vsock/test: check also expected errno on sigpipe test

In the sigpipe test, we expect send() to fail, but we do not check if
send() fails with the errno we expect (EPIPE).

Add this check and repeat the send() in case of EINTR as we do in other
tests.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20250514141927.159456-4-sgarzare@redhat.com


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