vtc_http2: Restrict SETTINGS frames to stream 0

In the unlikely event that we ever need actual coverage for this case,
we can handle it better than just failing the test case. In that regard,
as long as varnishtest itself can send SETTINGS frame on a wrong stream
to test varnishd's behavior, we should be fine.
Signed-off-by: Dridi Boukelmoune <dridi.boukelmoune@gmail.com>

bin/varnishtest/vtc_http2.c

@@ -607,6 +607,10 @@ parse_settings(const struct stream *s, struct frame *f)
 		vtc_fatal(s->vl,
 		    "Size should be a multiple of 6, but isn't (%d)", f->size);
 
+	if (s->id != 0)
+		vtc_fatal(s->vl,
+		    "Setting frames should only be on stream 0, but received on stream: %d", s->id);
+
 	for (u = 0; u <= SETTINGS_MAX; u++)
 		f->md.settings[u] = NAN;