test: refactor test-net-keepalive.js

- Replace require() vars with const.
- Replace assert.equal() with assert.strictEqual().
- Add common.mustCall() to the setTimeout() callback.

PR-URL: https://github.com/nodejs/node/pull/9995
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

test/parallel/test-net-keepalive.js

@@ -1,20 +1,20 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
-var net = require('net');
+const common = require('../common');
+const assert = require('assert');
+const net = require('net');
 
 var serverConnection;
 var clientConnection;
 var echoServer = net.createServer(function(connection) {
   serverConnection = connection;
-  setTimeout(function() {
+  setTimeout(common.mustCall(function() {
     // make sure both connections are still open
-    assert.equal(serverConnection.readyState, 'open');
-    assert.equal(clientConnection.readyState, 'open');
+    assert.strictEqual(serverConnection.readyState, 'open');
+    assert.strictEqual(clientConnection.readyState, 'open');
     serverConnection.end();
     clientConnection.end();
     echoServer.close();
-  }, common.platformTimeout(100));
+  }, 1), common.platformTimeout(100));
   connection.setTimeout(0);
   assert.notEqual(connection.setKeepAlive, undefined);
   // send a keepalive packet after 50 ms