Close test connection

For safety, closing the database aborts transaction managers
associated with open connections.  In explicit mode, this can lead to
NoTransaction errors.

It's likely a bug for DB objects to abort transactons on
close.  This needs more thought.

src/ZODB/tests/testConnection.py

@@ -1341,6 +1341,7 @@ class TestConnection(unittest.TestCase):
         self.assertEqual(len(syncs), 2)
         conn.transaction_manager.abort()
         self.assertEqual(len(syncs), 2)
+        conn.close()
         db.close()
 
         # For reference, in non-explicit mode:
@@ -1351,6 +1352,8 @@ class TestConnection(unittest.TestCase):
         self.assertEqual(len(syncs), 3)
         conn.transaction_manager.abort()
         self.assertEqual(len(syncs), 4)
+        conn.close()
+        db.close()
 
 class StubDatabase: