Stabilize test

bin/varnishtest/tests/c00069.vtc

 varnishtest "Test resp.is_streaming"
 
+barrier b1 sock 2
+barrier b2 sock 2
+
 server s1 {
 	rxreq
 	txresp -nolen -hdr "Content-Length: 10"
-	delay 1
+	barrier b1 sync
+	barrier b2 sync
 	send "1234567890"
 } -start
 
 varnish v1 -vcl+backend {
+	import vtc;
+
 	sub vcl_recv {
 		if (req.url == "/synth") {
 			return(synth(200, "OK"));
 		}
 	}
+	sub vcl_backend_response {
+		vtc.barrier_sync("${b1_sock}");
+		return (deliver);
+	}
 	sub vcl_synth {
 		set resp.http.streaming = resp.is_streaming;
 	}
 	sub vcl_deliver {
+		if (obj.hits == 0) {
+			vtc.barrier_sync("${b2_sock}");
+		}
 		set resp.http.streaming = resp.is_streaming;
 	}
 } -start
@@ -36,4 +49,3 @@ client c1 {
 	rxresp
 	expect resp.http.streaming == "false"
 } -run
-