exchange: fix a race condition in TestImportError
exchange: fix a race condition in TestImportError

lance2088/qor,shaunstanislaus/qor,yanzay/qor,insionng/qor,jmptrader/qor,codevlabs/qor,qor/qor,gavinzhs/qor,shidao-fm/qor,strogo/qor,jmptrader/qor,shidao-fm/qor,codedogfish/qor,youprofit/qor,insionng/qor,qor/qor,yanzay/qor,amirrpp/qor,lance2088/qor,codevlabs/qor,qor/qor,strogo/qor,codedogfish/qor,trigrass2/qor,youprofit/qor,gavinzhs/qor,amirrpp/qor,trigrass2/qor,shaunstanislaus/qor