Stabilize c91

There was a race with the reuse of s1 and server -dispatch was the
simplest way to also circumvent the race where varnish could retry
before s1 would reach the next accept action.

While at it, add coverage for the 503 case.

bin/varnishtest/tests/c00091.vtc

 varnishtest	"vcl_backend_response{} retry with a UDS backend"
 
-server s1 -listen "${tmpdir}/s1.sock" {
+server s0 -listen "${tmpdir}/s1.sock" {
 	rxreq
-	txresp -hdr "foo: 1"
-	accept
-	rxreq
-	txresp -hdr "foo: 2"
-} -start
+	txresp -hdr "connection: close"
+} -dispatch
 
 varnish v1 -vcl+backend {
 	sub vcl_recv { return (pass); }
 	sub vcl_backend_response {
-		set beresp.http.bar = bereq.retries;
-		if (beresp.http.foo != bereq.http.stop) {
+		set beresp.http.retries = bereq.retries;
+		if (bereq.http.stop != beresp.http.retries) {
 			return (retry);
 		}
 	}
 } -start
 
-varnish v1 -cliok "param.set debug +syncvsl"
-
 client c1 {
 	txreq -hdr "stop: 2"
 	rxresp
-	expect resp.http.foo == 2
+	expect resp.status == 200
+	expect resp.http.retries == 2
 } -run
 
-delay .1
-
-server s1 -listen "${tmpdir}/s1.sock" {
-	rxreq
-	txresp -hdr "foo: 1"
-	accept
-	rxreq
-	txresp -hdr "foo: 2"
-	accept
-	rxreq
-	txresp -hdr "foo: 3"
-} -start
-
 varnish v1 -cliok "param.set max_retries 2"
 
-client c1 {
+client c2 {
+	txreq -hdr "stop: 2"
+	rxresp
+	expect resp.status == 200
+	expect resp.http.retries == 2
+} -run
+
+client c3 {
 	txreq -hdr "stop: 3"
 	rxresp
-	expect resp.http.foo == 3
+	expect resp.status == 503
+	expect resp.http.retries == <undef>
 } -run
 
-# XXX: Add a test which exceeds max_retries and gets 503 back
+varnish v1 -expect backend_conn == 9