test: repl-persistent-history is no longer flaky

The race conditions were fixed in
286ef1da

PR-URL: https://github.com/nodejs/node/pull/3437


Reviewed By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>