test: refactor test-crypto-hash-stream-pipe

Change var to const.

PR-URL: nodejs#10055
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

test/parallel/test-crypto-hash-stream-pipe.js

@@ -1,17 +1,18 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
+const common = require('../common');
 
 if (!common.hasCrypto) {
   common.skip('missing crypto');
   return;
 }
-var crypto = require('crypto');
 
-var stream = require('stream');
-var s = new stream.PassThrough();
-var h = crypto.createHash('sha1');
-var expect = '15987e60950cf22655b9323bc1e281f9c4aff47e';
+const assert = require('assert');
+const crypto = require('crypto');
+
+const stream = require('stream');
+const s = new stream.PassThrough();
+const h = crypto.createHash('sha1');
+const expect = '15987e60950cf22655b9323bc1e281f9c4aff47e';
 
 s.pipe(h).on('data', common.mustCall(function(c) {
   assert.strictEqual(c, expect);