Ensure history database is closed in test

Hopefully fixes the last Windows test on Python 3.3

IPython/core/tests/test_history.py

@@ -136,6 +136,8 b' def test_history():'
         finally:
             # Ensure saving thread is shut down before we try to clean up the files
             ip.history_manager.save_thread.stop()
+            # Forcibly close database rather than relying on garbage collection
+            ip.history_manager.db.close()
             # Restore history manager
             ip.history_manager = hist_manager_ori