test: add failing test for #13696

It seems that the http module does not stream the initial few chunks of
a streaming http response in real-time. This has caused issues when
using bun with testcontainers.

While I am not qualified to locate and resolve the root cause of this
issue, I can provide a breaking test, in the hopes it will help somebody
else locate and fix the issue.

test/js/node/http/node-http.test.ts

@@ -133,6 +133,51 @@ describe("node:http", () => {
       }
     });
 
+    it("response body streaming is immediate (#13696)", async () => {
+      const totalChunks = 10;
+      const spacing = 50;
+      const acceptableDelay = 20;
+    
+      try {
+        var server = createServer(async (req, res) => {
+          res.writeHead(200, { "Content-Type": "text/plain" });
+          for (let i = 0; i < totalChunks; i++) {
+            res.write(new Date().getTime().toString() + "\n");
+    
+            if (i + 1 < totalChunks) await Bun.sleep(spacing);
+          }
+          res.end();
+        });
+        const url = await listen(server);
+        const res = await fetch(url);
+        const reader = res.body.getReader();
+        const decoder = new TextDecoder();
+    
+        let receivedChunks = 0;
+        while (true) {
+          const { done, value } = await reader.read();
+          if (done) break;
+    
+          receivedChunks++;
+    
+          // Verify that chunks are not held up longer than necessary
+          // at the receiver. This is likely to be in the single digits.
+          //
+          // #13696: Bun would delay the initial chunks and then send multiple
+          // chunks before real-time streaming started working.
+          expect(
+            new Date().getTime() - parseInt(decoder.decode(value).trimEnd())
+          ).toBeLessThan(acceptableDelay);
+        }
+    
+        // Verify that the correct number of chunks were sent (in case server
+        // decides to send no chunks at all).
+        expect(receivedChunks).toEqual(totalChunks);
+      } finally {
+        server.close();
+      }
+    });
+
     it("listen should return server", async () => {
       const server = createServer();
       const listenResponse = server.listen(0);