test/vsock: shutdowned socket test

This adds two tests for 'shutdown()' call. It checks that SIGPIPE is
sent when MSG_NOSIGNAL is not set and vice versa. Both flags SHUT_WR
and SHUT_RD are tested.

Signed-off-by: Arseniy Krasnov <avkrasnov@salutedevices.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>