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: #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>

tools/test.py

@@ -1578,7 +1578,7 @@ def Main():
         }
         test_list = root.ListTests([], path, context, arch, mode)
         unclassified_tests += test_list
-        (cases, unused_rules, all_outcomes) = (
+        (cases, unused_rules, _) = (
             config.ClassifyTests(test_list, env))
         if globally_unused_rules is None:
           globally_unused_rules = set(unused_rules)