test/http2: Add custom-response endpoint for flexible e2e testing

Add a `/custom-response` endpoint that returns a configurable response
via the `CUSTOM_RESPONSE` environment variable. Enable flexible e2e
testing by allowing customised backend responses to simulate various
routing and behaviour scenarios without modifying the application or
rebuilding the container.

test/http2/server.go

@@ -24,6 +24,7 @@ func lookupEnv(key, defaultVal string) string {
 func Serve() {
 	crtFile := lookupEnv("TLS_CRT", defaultTLSCrt)
 	keyFile := lookupEnv("TLS_KEY", defaultTLSKey)
+	customResponse := lookupEnv("CUSTOM_RESPONSE", "custom response")
 
 	http.HandleFunc("/", func(w http.ResponseWriter, req *http.Request) {
 		fmt.Fprint(w, req.Proto)
@@ -33,6 +34,10 @@ func Serve() {
 		fmt.Fprint(w, "ready")
 	})
 
+	http.HandleFunc("/custom-response", func(w http.ResponseWriter, req *http.Request) {
+		fmt.Fprint(w, customResponse)
+	})
+
 	go func() {
 		port := lookupEnv("HTTP_PORT", defaultHTTPPort)
 		log.Printf("Listening on port %v\n", port)