test: fix flaky test-http-server-keepalive-req-gc

Use `server` to keep the event loop open until the `ongc` listener runs.

PR-URL: #29347
Fixes: #29344
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>

test/parallel/test-http-server-keepalive-req-gc.js

@@ -16,7 +16,7 @@ if (common.isWindows) {
 
 let client;
 const server = createServer(common.mustCall((req, res) => {
-  onGC(req, { ongc: common.mustCall() });
+  onGC(req, { ongc: common.mustCall(() => { server.close(); }) });
   req.resume();
   req.on('end', common.mustCall(() => {
     setImmediate(() => {
@@ -27,8 +27,6 @@ const server = createServer(common.mustCall((req, res) => {
   res.end('hello world');
 }));
 
-server.unref();
-
 server.listen(0, common.mustCall(() => {
   client = connect(server.address().port);