tools: allow running test.py without configuring

If config.gypi isn't defined, assume Node was build the default way,
i.e. with the inspector.

PR-URL: https://github.com/nodejs/node/pull/16621
Refs: https://github.com/nodejs/node/pull/16436#issuecomment-340115175


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>