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/expected/all-window-properties.txt

@@ -408,6 +408,7 @@ __finishTest
 __preventMultipleTestFunctions
 animationFrame
 asyncTest
+httpTestServer
 printElement
 println
 promiseTest

Tests/LibWeb/Text/expected/test-http-test-server.txt

@@ -0,0 +1 @@
+PASS

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;
+}

Tests/LibWeb/Text/input/test-http-test-server.html

@@ -0,0 +1,29 @@
+<script src="./include.js"></script>
+<script>
+    asyncTest(async (done) => {
+        try {
+            const httpServer = httpTestServer();
+            const url = await httpServer.createEcho("GET", "/test-http-test-server", {
+                status: 200,
+                headers: {
+                    "Access-Control-Allow-Origin": "*",  // necessary when running test using file://...
+                    "Access-Control-Expose-Headers": "X-Custom-Header", // tells the browser which headers to expose to JS
+                    "Content-Type": "text/plain",
+                    "X-Custom-Header": "well hello friends",
+                },
+                body: "hello world!",
+            });
+            const result = await fetch(url);
+            const headers = result.headers;
+
+            if (headers.get("X-Custom-Header") === "well hello friends") {
+                println("PASS");
+            } else {
+                println("FAIL");
+            }
+        } catch (err) {
+            println("FAIL - " + err);
+        }
+        done();
+    });
+</script>