test: ensure that all worker servers are ready

Wait for the `'listening'` message of all workers before creating the
first connection. This fixes an `EMFILE` error that is raised on Windows
when running the following command

```
python tools/test.py -J --repeat=1000 parallel/test-tls-ticket-cluster
```

test/parallel/test-tls-ticket-cluster.js

@@ -33,10 +33,10 @@ const workerCount = 4;
 const expectedReqCount = 16;
 
 if (cluster.isPrimary) {
+  let listeningCount = 0;
   let reusedCount = 0;
   let reqCount = 0;
   let lastSession = null;
-  let shootOnce = false;
   let workerPort = null;
 
   function shoot() {
@@ -73,9 +73,8 @@ if (cluster.isPrimary) {
       console.error('[primary] got %j', msg);
       if (msg === 'reused') {
         ++reusedCount;
-      } else if (msg === 'listening' && !shootOnce) {
-        workerPort = port || workerPort;
-        shootOnce = true;
+      } else if (msg === 'listening' && ++listeningCount === workerCount) {
+        workerPort = port;
         shoot();
       }
     });