test: polish test-net-better-error-messages-listen

Cleans up test-net-better-error-messages-list.js with following:
- var -> const
- assert.equal -> assert.strictEqual

PR-URL: nodejs#10087
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

test/parallel/test-net-better-error-messages-listen.js

@@ -1,12 +1,12 @@
 '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 server = net.createServer(common.fail);
+const server = net.createServer(common.fail);
 server.listen(1, '1.1.1.1', common.fail);
 server.on('error', common.mustCall(function(e) {
-  assert.equal(e.address, '1.1.1.1');
-  assert.equal(e.port, 1);
-  assert.equal(e.syscall, 'listen');
+  assert.strictEqual(e.address, '1.1.1.1');
+  assert.strictEqual(e.port, 1);
+  assert.strictEqual(e.syscall, 'listen');
 }));