test: fix lint error

Fix lint error introduced 03550a5c1e. Unused module `fs` is removed.

PR-URL: https://github.com/nodejs/node/pull/16145
Ref: https://github.com/nodejs/node/pull/15832#issuecomment-335808467


Reviewed-By: Bradley Farias <bradley.meck@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>