Fix DescendantsTrackerTest when ran in isolation

Followup: #49108

DescendantsTracker need to work whether the `Class#descendants` core
ext is loaded or not. I missed that in the previous PR.

activesupport/lib/active_support/descendants_tracker.rb

@@ -107,7 +107,7 @@ def descendants(klass)
       end
 
       def descendants
-        subclasses = self.subclasses
+        subclasses = DescendantsTracker.reject!(self.subclasses)
         subclasses.concat(subclasses.flat_map(&:descendants))
       end
     else