test: fixed FIXME in test-repl-persistent-history

When NODE_REPL_HISTORY isn't defined `repl` defaults to temporary file
This prevents the temporary file from being cleared and removes check
on fixture

Refs: https://github.com/nodejs/node/issues/4640
PR-URL: https://github.com/nodejs/node/pull/8756


Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>