vtc: Make t02011.vtc failures more visible

Seeing a test failure on vtest I think there might be a race here that I
hope to make more explicit next time it occurs.

bin/varnishtest/tests/t02011.vtc

@@ -32,7 +32,7 @@ varnish v1 -vcl+backend {
 
 	sub vcl_recv {
 		if (req.http.should == "reset") {
-			return (fail);
+			vtc.panic("Expected stream reset REFUSED_STREAM");
 		}
 	}