test: deflake test-http2-empty-frame-without-eof

It may happen that the data in `emptyframe.http2` reaches the client
even before the client has started sending the request, causing an
`ERR_HTTP2_STREAM_ERROR` instead. Fix this by making sure the frame is
not sent until some data reaches the server.

PR-URL: #45212
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Mohammed Keyvanzadeh <[email protected]>
Reviewed-By: Juan José Arboleda <[email protected]>

Author: santigimeno

test/parallel/test-http2-empty-frame-without-eof.js

@@ -10,7 +10,9 @@ const { once } = require('events');
 async function main() {
   const blobWithEmptyFrame = readSync('emptyframe.http2');
   const server = net.createServer((socket) => {
-    socket.end(blobWithEmptyFrame);
+    socket.once('data', () => {
+      socket.end(blobWithEmptyFrame);
+    });
   }).listen(0);
   await once(server, 'listening');