test: refactor test-fs-symlink-dir-junction

* var -> const
* assert.equal() -> assert.strictEqual()

PR-URL: https://github.com/nodejs/node/pull/9928


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>