Test calling NilmDB.close twice to improve coverage

tests/test_nilmdb.py

@@ -33,6 +33,7 @@ class Test00Nilmdb(object):  # named 00 so it runs first
         db.close()
         db = nilmdb.server.NilmDB(testdb)
         db.close()
+        db.close()
 
         # test timer, just to get coverage
         capture = io.StringIO()