add smoke test for BatchStatement.__len__
add smoke test for BatchStatement.__len__

datastax/python-driver,mambocab/python-driver,thelastpickle/python-driver,thelastpickle/python-driver,mambocab/python-driver,coldeasy/python-driver,datastax/python-driver,vipjml/python-driver,coldeasy/python-driver,vipjml/python-driver