Add tear down to APIv4 tests (#5250)

* Add tear down to APIv4 tests

* Defer tear downs

store/sql_post_store.go

@@ -409,6 +409,23 @@ func (s SqlPostStore) PermanentDeleteByUser(userId string) StoreChannel {
 	return storeChannel
 }
 
+func (s SqlPostStore) PermanentDeleteByChannel(channelId string) StoreChannel {
+	storeChannel := make(StoreChannel, 1)
+
+	go func() {
+		result := StoreResult{}
+
+		if _, err := s.GetMaster().Exec("DELETE FROM Posts WHERE ChannelId = :ChannelId", map[string]interface{}{"ChannelId": channelId}); err != nil {
+			result.Err = model.NewLocAppError("SqlPostStore.PermanentDeleteByChannel", "store.sql_post.permanent_delete_by_channel.app_error", nil, "channel_id="+channelId+", "+err.Error())
+		}
+
+		storeChannel <- result
+		close(storeChannel)
+	}()
+
+	return storeChannel
+}
+
 func (s SqlPostStore) GetPosts(channelId string, offset int, limit int, allowFromCache bool) StoreChannel {
 	storeChannel := make(StoreChannel, 1)