Add an HTTP/2 test to e23.vtc, such as it is.

See comments in the test about unfortunate limits using vtc to
test gzipped H2 responses.

src/tests/e00023.vtc

@@ -104,3 +104,23 @@ client c1 {
 
 client c1 -run
 varnish v1 -expect esi_errors == 0
+
+## HTTP/2
+
+varnish v1 -cliok "param.set feature +http2"
+
+## vtc evidently does not allow the gunzip command within an H2
+## stream. So AFAICT we don't have a way to verify the response body
+## contents. I'm not sure if the length of the compressed response is
+## necessarily predictable; if not, then we can't even rely on
+## that. And the contents of an H2 response body are not even shown in
+## the vtc log. This is, um, unfortunate.
+
+client c1 {
+	stream 1 {
+		txreq -hdr accept-encoding gzip
+		rxresp
+		expect resp.http.content-encoding == gzip
+		expect resp.status == 200
+	} -run
+} -run