test: remove unused vars in ChildProcess tests

In addition to removing unused vars, this also fixes an instance where
booleans were set presumably to check something but then never used.
This now confirms that the events that were setting the booleans are
fired.

PR-URL: https://github.com/nodejs/node/pull/4425


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