MIXED_DML_OPERATION, DML operation on setup object is not permitted after you have updated a non-setup object (or vice versa): User, original object: Account

This error is so common when attempting to create user and other objects records in a single transaction in apex. 

Workaround in apex class/trigger : use future method for creating user when encountered the error

Workaround in test class : don't try creating a new user data, instead use <System.runAs(new User(profileId =<required_Profile>))>

Since the test class needs to be Annotated SeeAllData=false, developer should try creating a TestUtil class to fetch the user data and call the same in the test class, code-snippet as below : 

TestUtil.cls : 
public static fetchRequiredProfileUser(){
 return [SELECT ID, Name FROM User WHERE ProfileId ='**********'];
}

Test Class :
@isTest private void useTest(){
 System.runAs(new User(TestUtil.fetchRequiredProfileUser())[0]);
}