tools: make run-valgrind.py useful

Node.js does not clean up on exit so don't complain about memory leaks
but do complain about invalid memory access.  In the future we may want
to add a cleanup-on-exit flag or put together a suppression list.

PR-URL: https://github.com/nodejs/node/pull/9520


Reviewed By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>