vsock_test: POLLIN + SO_RCVLOWAT test

This adds test to check, that when poll() returns POLLIN, POLLRDNORM bits,
next read call won't block.

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