Fix const `Post` leakage in `test/cases/base_test.rb`

https://buildkite.com/rails/rails/builds/60601#81daf2ee-9259-42e5-983d-c57d66499b76/928-939

activerecord/test/cases/base_test.rb

@@ -1215,6 +1215,8 @@ def test_marshal_between_processes
       wr.close
       assert Marshal.load rd.read
       rd.close
+    ensure
+      self.class.send(:remove_const, "Post") if self.class.const_defined?("Post", false)
     end
   end