Do a "backend.list" to try to provoke any pending panics better,

this CLI command goes to the varnish worker process if it is
running and gives an error (which we ignore) if it isn't.

bin/varnishtest/vtc_varnish.c

@@ -576,11 +576,13 @@ varnish_wait(struct varnish *v)
 	void *p;
 	int status, r;
 	struct rusage ru;
+	char *resp;
 
 	if (v->cli_fd < 0)
 		return;
 	if (vtc_error)
 		(void)sleep(1);	/* give panic messages a chance */
+	varnish_ask_cli(v, "backend.list", &resp);
 	varnish_stop(v);
 	vtc_log(v->vl, 2, "Wait");
 	AZ(close(v->cli_fd));