test: fix flaky test-dgram-exclusive-implicit-bind

test-dgram-exclusive-implicit-bind is written assuming that dgram
messages are received with 100% reliability. While missing a dgram
message sent to localhost is rare, we do see it as evidenced by CI
failures from time to time.

The test has been rewritten to send dgram messages over and over until
the test requirements have been met.

Additional incidental refactoring includes:

* var -> const
* use of common.mustCall() instead of exit listener + boolean

test/parallel/test-dgram-exclusive-implicit-bind.js

@@ -20,10 +20,10 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
 // USE OR OTHER DEALINGS IN THE SOFTWARE.
 
-var common = require('../common');
-var assert = require('assert');
-var cluster = require('cluster');
-var dgram = require('dgram');
+const common = require('../common');
+const assert = require('assert');
+const cluster = require('cluster');
+const dgram = require('dgram');
 
 // Without an explicit bind, send() causes an implicit bind, which always
 // generate a unique per-socket ephemeral port. An explicit bind to a port
@@ -40,17 +40,21 @@ var dgram = require('dgram');
 // with ENOTSUP.
 
 if (cluster.isMaster) {
-  var pass;
   var messages = 0;
-  var ports = {};
-
-  process.on('exit', function() {
-    assert.strictEqual(pass, true);
-  });
+  const ports = {};
+  const pids = [];
 
   var target = dgram.createSocket('udp4');
 
+  const done = common.mustCall(function() {
+    cluster.disconnect();
+    target.close();
+  });
+
   target.on('message', function(buf, rinfo) {
+    if (pids.includes(buf.toString()))
+      return;
+    pids.push(buf.toString());
     messages++;
     ports[rinfo.port] = true;
 
@@ -63,12 +67,6 @@ if (cluster.isMaster) {
       assert.strictEqual(Object.keys(ports).length, 3);
       done();
     }
-
-    function done() {
-      pass = true;
-      cluster.disconnect();
-      target.close();
-    }
   });
 
   target.on('listening', function() {
@@ -85,7 +83,12 @@ if (cluster.isMaster) {
   return;
 }
 
-var source = dgram.createSocket('udp4');
+const source = dgram.createSocket('udp4');
+var timer;
+
+source.on('close', function() {
+  clearTimeout(timer);
+});
 
 if (process.env.BOUND === 'y') {
   source.bind(0);
@@ -96,4 +99,12 @@ if (process.env.BOUND === 'y') {
   source.unref();
 }
 
-source.send(Buffer.from('abc'), 0, 3, common.PORT, '127.0.0.1');
+function send() {
+  const buf = Buffer.from(process.pid.toString());
+  timer = setTimeout(function() {
+    source.send(buf, common.PORT, '127.0.0.1', send);
+  }, 1);
+  timer.unref();
+}
+
+send();