test: clean up test-child-process-exec-cwd.js

- Changed `assert.ok()` to `assert.strictEqual()`.
- Changed `var` to `const` where possible.

PR-URL: nodejs#9231
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>

test/parallel/test-child-process-exec-cwd.js

@@ -1,7 +1,7 @@
 'use strict';
 const common = require('../common');
-var assert = require('assert');
-var exec = require('child_process').exec;
+const assert = require('assert');
+const exec = require('child_process').exec;
 
 var pwdcommand, dir;
 
@@ -15,5 +15,5 @@ if (common.isWindows) {
 
 exec(pwdcommand, {cwd: dir}, common.mustCall(function(err, stdout, stderr) {
   assert.ifError(err);
-  assert.ok(stdout.indexOf(dir) == 0);
+  assert.strictEqual(stdout.indexOf(dir), 0);
 }));