const word = "テスト"
const query = ({test: "/" + word + "/"})
testDB.find(query).function(err, item) {
 //hogehoge
}

var word = "テスト"
var query = ({test: new RegExp(".*" + word + ".*" , "i")})
testDB.find(query).function(err, item) {
 //hogehoge
}