test: add ES6 and strictEqual to test-fs-truncate

- use const and let for variables
- replace assert.equal with assert.strictEqual

PR-URL: https://github.com/nodejs/node/pull/10167


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>