test: improve cluster-disconnect-handles test

This commit fixes two issues in test-cluster-disconnect-handles:

1. If the master's TCP connection to the worker fails, the worker
process stays alive and causes many other tests that use the same
common port number to also fail (with EADDRINUSE).

2. One particular problem that can cause the master's TCP connection
to fail is attempting an IPv6 connection to the worker when no IPv6
network interfaces are available.

PR-URL: #4084
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/test-cluster-disconnect-handles.js

@@ -23,6 +23,7 @@ cluster.schedulingPolicy = cluster.SCHED_RR;
 // is to make sure the connection is still sitting in the master's
 // pending handle queue.
 if (cluster.isMaster) {
+  let isKilling = false;
   const handles = require('internal/cluster').handles;
   // FIXME(bnoordhuis) lib/cluster.js scans the execArgv arguments for
   // debugger flags and renumbers any port numbers it sees starting
@@ -68,11 +69,30 @@ if (cluster.isMaster) {
     }));
   }));
   process.on('exit', () => assert.deepStrictEqual(handles, {}));
+  process.on('uncaughtException', function(ex) {
+    // Make sure we clean up so as not to leave a stray worker process running
+    // if we encounter a connection or other error
+    if (!worker.isDead()) {
+      if (!isKilling) {
+        isKilling = true;
+        worker.once('exit', function() {
+          throw ex;
+        });
+        worker.process.kill();
+      }
+      return;
+    }
+    throw ex;
+  });
 } else {
   const server = net.createServer(socket => socket.pipe(socket));
-  server.listen(() => {
+  const cb = () => {
     process.send(['listening', server.address()]);
     debugger;
-  });
+  };
+  if (common.hasIPv6)
+    server.listen(cb);
+  else
+    server.listen(0, common.localhostIPv4, cb);
   process.on('disconnect', process.exit);
 }