test: fix tests re: #8073

Author: vkarpov15

lib/model.js

@@ -29,6 +29,7 @@ const applyWriteConcern = require('./helpers/schema/applyWriteConcern');
 const assignVals = require('./helpers/populate/assignVals');
 const castBulkWrite = require('./helpers/model/castBulkWrite');
 const discriminator = require('./helpers/model/discriminator');
+const each = require('./helpers/each');
 const getDiscriminatorByValue = require('./queryhelpers').getDiscriminatorByValue;
 const getModelsMapForPopulate = require('./helpers/populate/getModelsMapForPopulate');
 const immediate = require('./helpers/immediate');
@@ -3381,7 +3382,7 @@ Model.bulkWrite = function(ops, options, callback) {
   const validations = ops.map(op => castBulkWrite(this, op, options));
 
   return utils.promiseOrCallback(callback, cb => {
-    parallel(validations, error => {
+    each(validations, (fn, cb) => fn(cb), error => {
       if (error) {
         return cb(error);
       }