test: refactor test-timers-this

* use common.mustCall() and eliminate exit handler
* provide timer durtion of 1ms where previously omitted
* var -> const

PR-URL: https://github.com/nodejs/node/pull/10315


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>