test: remove duplicate fork module import

`fork` is imported twice in a row. Remove duplication.

PR-URL: https://github.com/nodejs/node/pull/4634


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>