test: refactor test-fs-empty-readStream

Refactor test to remove unnecessary booleans and one unnecesary timer.
Instead, throw Error objects where appropriate and rely on
common.mustCall().

The timer seemed to be the source of an issue when parallelizing tests.

Ref: https://github.com/nodejs/node/pull/4476#issuecomment-168080875
PR-URL: https://github.com/nodejs/node/pull/4490


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>