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

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

PR-URL: https://github.com/nodejs/node/pull/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>