Stop using httpbin.org for testing (now very unreliable)

New solution is just a quick hack too, but at least it's a bit more
stable - httpbin.org just times out most of the time now.

Author: pimterry

test-app/src/main/java/tech/httptoolkit/testapp/Main.java

@@ -41,7 +41,7 @@ public static void main(String[] args) throws Exception {
         String pid = runtimeName.split("@")[0];
         System.out.println("PID: " + pid); // Purely for convenient manual attachment to this process
 
-        String url = "https://httpbin.org/404/"; // Always returns a 404, quelle surprise
+        String url = "https://example.com/404/"; // Always returns a 404
         ExecutorService executor = Executors.newCachedThreadPool();
 
         while (true) {