LibWeb/Tests: Added example test for how to use http-test-server.py

The example shows how to write a test that depends on custom HTTP
headers in the response. This will be useful for testing browser JS
that depends on how Ladybird processes response headers, eg CORS
headers like Access-Control-Allow-Origin and others.

Tests/LibWeb/Text/input/include.js

@@ -98,3 +98,31 @@ function promiseTest(f) {
         });
     });
 }
+
+class HTTPTestServer {
+    constructor(baseURL) {
+        this.baseURL = baseURL;
+    }
+    async createEcho(method, path, options) {
+        const result = await fetch(`${this.baseURL}/create`, {
+            method: "POST",
+            headers: {
+                "Content-Type": "application/json",
+            },
+            body: JSON.stringify({ ...options, method, path }),
+        });
+        if (!result.ok) {
+            throw new Error("Error creating echo: " + result.statusText);
+        }
+        return `${this.baseURL}${path}`;
+    }
+    getStaticURL(path) {
+        return `${this.baseURL}/static/${path}`;
+    }
+}
+
+// FIXME: Get the port from internals
+const __httpTestServer = new HTTPTestServer("http://localhost:8123");
+function httpTestServer() {
+    return __httpTestServer;
+}