test: refactor test-http-dns-error

Replace var with const and use strictEqual().

PR-URL: nodejs#10062
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

test/parallel/test-http-dns-error.js

@@ -1,16 +1,16 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
+const common = require('../common');
+const assert = require('assert');
 
-var http = require('http');
+const http = require('http');
 
 if (common.hasCrypto) {
   var https = require('https');
 } else {
   common.skip('missing crypto');
 }
 
-var host = '*'.repeat(256);
+const host = '*'.repeat(256);
 
 function do_not_call() {
   throw new Error('This function should not have been called.');
@@ -20,15 +20,15 @@ function test(mod) {
 
   // Bad host name should not throw an uncatchable exception.
   // Ensure that there is time to attach an error listener.
-  var req1 = mod.get({host: host, port: 42}, do_not_call);
+  const req1 = mod.get({host: host, port: 42}, do_not_call);
   req1.on('error', common.mustCall(function(err) {
-    assert.equal(err.code, 'ENOTFOUND');
+    assert.strictEqual(err.code, 'ENOTFOUND');
   }));
   // http.get() called req1.end() for us
 
-  var req2 = mod.request({method: 'GET', host: host, port: 42}, do_not_call);
+  const req2 = mod.request({method: 'GET', host: host, port: 42}, do_not_call);
   req2.on('error', common.mustCall(function(err) {
-    assert.equal(err.code, 'ENOTFOUND');
+    assert.strictEqual(err.code, 'ENOTFOUND');
   }));
   req2.end();
 }