vtc: New stream.peer_window variable for h2 checks

Closes #3681
Signed-off-by: Dridi Boukelmoune <dridi.boukelmoune@gmail.com>

bin/varnishtest/tests/a02006.vtc

@@ -8,8 +8,20 @@ server s1 {
 		txcont -hdr "baz" "qux"
 		txdata -data "foo"
 		txdata -data "bar"
+		expect stream.peer_window == 65529
+		rxreq
+		txresp -bodylen 529
+		expect stream.peer_window == 65000
+		rxwinup
+		expect stream.peer_window == 65200
+	} -run
+	stream 0 {
+		expect stream.peer_window == 65000
+		rxwinup
+		expect stream.peer_window == 66000
 	} -run
 
+
 } -start
 
 client c1 -connect ${s1_sock} {
@@ -35,6 +47,15 @@ client c1 -connect ${s1_sock} {
 	stream 0 {
 		expect stream.window == 65529
 	} -run
+	stream 2 {
+		txreq
+		rxresp
+		txwinup -size 200
+	} -run
+	stream 0 {
+		txwinup -size 1000
+	} -run
+
 } -run
 
 server s1 -wait

bin/varnishtest/tests/a02010.vtc

@@ -2,17 +2,23 @@ varnishtest "Verify the initial window size"
 
 server s1 {
 	stream 0 {
+		expect stream.peer_window == 65535
 		rxsettings
 		txsettings -ack
 	} -run
 	stream 1 {
+		expect stream.peer_window == 128
 		rxreq
 		txresp -bodylen 100
+		expect stream.peer_window == 28
 	} -run
 	stream 0 {
 		rxsettings
 		txsettings -ack
 	} -run
+	stream 1 {
+		expect stream.peer_window == -36
+	} -run
 } -start
 
 client c1 -connect ${s1_sock} {

bin/varnishtest/vtc_http2.c

@@ -1124,7 +1124,11 @@ cmd_var_resolve(const struct stream *s, const char *spec, char *buf)
 	/* SECTION: stream.spec.zexpect.zstream Stream
 	 *
 	 * stream.window
-	 *	The current window size of the stream, or, if on stream 0,
+	 *	The current local window size of the stream, or, if on stream 0,
+	 *	of the connection.
+	 *
+	 * stream.peer_window
+	 *	The current peer window size of the stream, or, if on stream 0,
 	 *	of the connection.
 	 *
 	 * stream.weight
@@ -1138,6 +1142,11 @@ cmd_var_resolve(const struct stream *s, const char *spec, char *buf)
 		    (intmax_t)(s->id ? s->win_self : s->hp->h2_win_self->size));
 		return (buf);
 	}
+	if (!strcmp(spec, "stream.peer_window")) {
+		snprintf(buf, 20, "%jd",
+		    (intmax_t)(s->id ? s->win_peer : s->hp->h2_win_peer->size));
+		return (buf);
+	}
 	if (!strcmp(spec, "stream.weight")) {
 		if (s->id) {
 			snprintf(buf, 20, "%d", s->weight);