Merge pull request rails#49111 from Shopify/fix-isolated-descendants-tracker-test

Fix DescendantsTrackerTest when ran in isolation

Author: byroot

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