test: fix flaky http2-session-unref

It's possible for the connections to take too long and since the server
is already unrefed, the process will just exit. Instead adjust the test
so that server unref only happens after all sessions have been
successfuly established and unrefed. That still tests the same condition
but will not fail under load.
PR-URL: #20772Fixes: #20705Fixes: #20750Fixes: #20850
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: apapirovski

test/parallel/test-http2-session-unref.js

@@ -9,16 +9,20 @@ const common = require('../common');
 if(!common.hasCrypto)
 common.skip('missing crypto');
 consthttp2=require('http2');
+constCountdown=require('../common/countdown');
 constmakeDuplexPair=require('../common/duplexpair');
 
 constserver=http2.createServer();
 const{ clientSide, serverSide }=makeDuplexPair();
 
+constcounter=newCountdown(3,()=>server.unref());
+
 // 'session' event should be emitted 3 times:
 // - the vanilla client
 // - the destroyed client
 // - manual 'connection' event emission with generic Duplex stream
 server.on('session',common.mustCallAtLeast((session)=>{
+counter.dec();
 session.unref();
 },3));
 
@@ -54,4 +58,3 @@ server.listen(0, common.mustCall(() => {
 }
 }));
 server.emit('connection',serverSide);
-server.unref();