test: clean up test-timers-immediate

Clean up test-timers-immediate. Use of `let` also requires a tweak to
ESLint rules (but it's one that we should do as timers is pretty much
the reason it exists).

PR-URL: https://github.com/nodejs/node/pull/8857


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>