tools: rename unused variale in more pythonic way

The 'Main' function in tools/test.py file was using a variable named
``all_outcomes`` to store a value not being used. It is a best practice
to name unused variables, often return values of functions/methods (as
in this case) as ``_`` [1]. This just helps keep the code a bit cleaner
and avoid any silly mistakes.

PR-URL: https://github.com/nodejs/node/pull/16171
Refs: [1] https://stackoverflow.com/a/5477153


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>