test: do not fail SLOW tests if they are not slow

PR-URL: https://github.com/nodejs/node/pull/25868
Refs: https://github.com/nodejs/node/issues/25867
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gus Caplan <me@gus.host>

tools/test.py

@@ -1238,6 +1238,9 @@ class Configuration(object):
       outcomes = reduce(set.union, outcomes_list, set())
       unused_rules.difference_update(matches)
       case.outcomes = set(outcomes) or set([PASS])
+      # slow tests may also just pass.
+      if SLOW in case.outcomes:
+        case.outcomes.add(PASS)
       result.append(case)
     return result, unused_rules