test: refactor test-timers-linked-list.js

* check that empty linked lists have identical properties
* `assert.equal()` -> `assert.strictEqual()`
* `var` -> `const`

PR-URL: https://github.com/nodejs/node/pull/8193


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>