test: deflake test-http2-large-write-multiple-requests

If the server is not referenced, it might go away too soon
and the client may not get enough ends for it to close
itself, resulting a timeout.
This patch updates the test to simply close the server when
enough requests have been processed, and keep the server
referenced while the test is ongoing.

Drive-by: add more logs to facilitate debugging.
PR-URL: #51863
Refs: nodejs/reliability#791
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-http2-large-write-multiple-requests.js

@@ -3,6 +3,10 @@ const common = require('../common');
 if (!common.hasCrypto)
   common.skip('missing crypto');
 
+// This tests that the http2 server sends data early when it accumulates
+// enough from ongoing requests to avoid DoS as mitigation for
+// CVE-2019-9517 and CVE-2019-9511.
+// Added by https://github.com/nodejs/node/commit/8a4a193
 const fixtures = require('../common/fixtures');
 const assert = require('assert');
 const http2 = require('http2');
@@ -12,18 +16,20 @@ const content = fixtures.readSync('person-large.jpg');
 const server = http2.createServer({
   maxSessionMemory: 1000
 });
+let streamCount = 0;
 server.on('stream', (stream, headers) => {
   stream.respond({
     'content-type': 'image/jpeg',
     ':status': 200
   });
   stream.end(content);
+  console.log('server sends content', ++streamCount);
 });
-server.unref();
 
 server.listen(0, common.mustCall(() => {
   const client = http2.connect(`http://localhost:${server.address().port}/`);
 
+  let endCount = 0;
   let finished = 0;
   for (let i = 0; i < 100; i++) {
     const req = client.request({ ':path': '/' }).end();
@@ -32,8 +38,16 @@ server.listen(0, common.mustCall(() => {
       chunks.push(chunk);
     });
     req.on('end', common.mustCall(() => {
+      console.log('client receives content', ++endCount);
       assert.deepStrictEqual(Buffer.concat(chunks), content);
-      if (++finished === 100) client.close();
+
+      if (++finished === 100) {
+        client.close();
+        server.close();
+      }
     }));
+    req.on('error', (e) => {
+      console.log('client error', e);
+    });
   }
 }));