I am trying to write a test method for account custom sharing with Partner Portal user. But I am facing error while setting up u.IsActive = true; and u.IsPortalEnabled = true;  fields in test methods. Error as follows.

System.DmlException: Update failed. First exception on row 0 with id 005O0000000R2NTIA0; first error: 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: []

I understanding we can’t run DML on user and sharing object together(Set up and non setup objects). Wondered if there is any solution so that we can create partner portal user in a test method beside sele4cting this from the User object.    

@isTest

private class TestUpdateAccountShare_Trigger{

    public static testMethod void myTest()

    {

        Test.startTest();

    //Account created.

    Account acc = new Account(name='test');

    insert acc;

    //Contact created.   

    Contact cont = new Contact();

    cont.lastname = 'test';

    cont.phone = '555-555-5555';

    cont.Accountid =acc.id;

    insert cont;

    //Selects name and id partner user profile.

    Profile pp = [Select p.Name, p.Id From Profile p where Name LIKE '%Partner%'limit 1];

    //Partner user inserted.

    User u = new User(alias = 'port',

                email='portaluser@testorg.com',

                emailencodingkey='UTF-8',

                lastname='Testing',

                languagelocalekey='en_US',

                localesidkey='en_US',

                profileid = pp.Id,

                contactid = cont.Id,

                timezonesidkey='America/Los_Angeles',

                username='portaluser@testorg.com'

                );

    insert u;

    u.IsActive = true;

    u.IsPortalEnabled = true;

    update u;

        Test.stopTest();

    //Select partner portal user's id and account id from contacts.

    List<Contact> con = [Select id, Accountid from Contact where id =: u.ContactId];

    System.debug('User record' + con);

    //Account's record created.

    account accobj = new account(name='test');

    System.debug('User record' + accobj);

    insert accobj;

    }

}