test: updated tls-getcipher test

Changed var to ES6 syntax const/let.
Added common.mustCall function wrapper to all functions
called exactly once.
Changed assert.equal to assert.strictEqual.

PR-URL: https://github.com/nodejs/node/pull/9923
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-tls-getcipher.js

@@ -1,37 +1,37 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
+const common = require('../common');
 
 if (!common.hasCrypto) {
   common.skip('missing crypto');
   return;
 }
-var tls = require('tls');
+const assert = require('assert');
+const tls = require('tls');
 
-var fs = require('fs');
-var cipher_list = ['AES128-SHA256', 'AES256-SHA256'];
-var cipher_version_pattern = /TLS|SSL/;
-var options = {
+const fs = require('fs');
+const cipher_list = ['AES128-SHA256', 'AES256-SHA256'];
+const cipher_version_pattern = /TLS|SSL/;
+const options = {
   key: fs.readFileSync(common.fixturesDir + '/keys/agent2-key.pem'),
   cert: fs.readFileSync(common.fixturesDir + '/keys/agent2-cert.pem'),
   ciphers: cipher_list.join(':'),
   honorCipherOrder: true
 };
 
-var server = tls.createServer(options,
+const server = tls.createServer(options,
                                 common.mustCall(function(cleartextStream) {}));
 
-server.listen(0, '127.0.0.1', function() {
-  var client = tls.connect({
+server.listen(0, '127.0.0.1', common.mustCall(function() {
+  const client = tls.connect({
     host: '127.0.0.1',
     port: this.address().port,
     ciphers: cipher_list.join(':'),
     rejectUnauthorized: false
-  }, function() {
-    var cipher = client.getCipher();
-    assert.equal(cipher.name, cipher_list[0]);
+  }, common.mustCall(function() {
+    const cipher = client.getCipher();
+    assert.strictEqual(cipher.name, cipher_list[0]);
     assert(cipher_version_pattern.test(cipher.version));
     client.end();
     server.close();
-  });
-});
+  }));
+}));