Stabilise test cases

With the new request body data handling, Varnish changes behaviour
significantly wrt to stream window updates sent to the client. Window
updates will only be sent once the data is consumed by the client through
the request body VFP handling. Test cases that rely on receiving a window
update to sync the H/2 stream needs to be adopted.

bin/varnishtest/tests/r02679.vtc

@@ -22,7 +22,6 @@ client c1 {
 	stream 1 {
 		txreq -req POST -hdr "content-length" "31469" -nostrend
 		txdata -datalen 1550 -nostrend
-		rxwinup
 		txdata -datalen 16000 -nostrend
 		txdata -datalen 13919
 		rxresp

bin/varnishtest/tests/t02014.vtc

@@ -2,7 +2,13 @@ varnishtest "Exercise h/2 sender flow control code"
 
 barrier b1 sock 3 -cyclic
 
-server s1 -repeat 2 {
+server s1 {
+	rxreq
+	txresp -bodylen 66300
+} -start
+
+server s2 {
+	non_fatal
 	rxreq
 	txresp -bodylen 66300
 } -start
@@ -10,6 +16,12 @@ server s1 -repeat 2 {
 varnish v1 -vcl+backend {
 	import vtc;
 
+	sub vcl_backend_fetch {
+		if (bereq.method == "POST") {
+			set bereq.backend = s2;
+		}
+	}
+
 	sub vcl_deliver {
 		vtc.barrier_sync("${b1_sock}");
 	}
@@ -47,7 +59,7 @@ client c1 {
 	stream 0 -wait
 } -run
 
-client c1 {
+client c2 {
 	stream 0 {
 		barrier b1 sync
 	} -start
@@ -63,7 +75,7 @@ client c1 {
 	stream 0 -wait
 } -run
 
-client c1 {
+client c3 {
 	stream 0 {
 		barrier b1 sync
 		barrier b1 sync
@@ -79,7 +91,6 @@ client c1 {
 	stream 1 {
 		txreq -req "POST" -nostrend
 		txdata -data "ok"
-		rxwinup
 		txdata -data "fail"
 		rxrst
 		expect rst.err == STREAM_CLOSED