The Mixed DML error is generated – when a User performs DML actions on Setup and non-Setup Objects in the same transaction -because some sObjects affect the user's access to records in the org. ... For example, a user cannot update an account, and a user role, in a single transaction.

There are two ways to solve mixed DML error
1. System.runAs
2. @future
Example: Mixed DML Operations in System.runAs Blocks
This example shows how to enclose mixed DML operations within System.runAs blocks to avoid the mixed DML error. The System.runAs block runs in the current user’s context. It creates a test user with a role and a test account, which is a mixed DML operation.

@isTest
private class MixedDML {
static testMethod void mixedDMLExample() {
User u;
Account a;
User thisUser = [SELECT Id FROM User WHERE Id = :UserInfo.getUserId()];
// Insert account as current user
System.runAs (thisUser) {
Profile p = [SELECT Id FROM Profile WHERE Name='Standard User'];
UserRole r = [SELECT Id FROM UserRole WHERE Name='COO'];
u = new User(alias = 'jsmith', email='jsmith@acme.com',
emailencodingkey='UTF-8', lastname='Smith',
languagelocalekey='en_US',
localesidkey='en_US', profileid = p.Id, userroleid = r.Id,
timezonesidkey='America/Los_Angeles',
username='jsmith@acme.com');
insert u;
a = new Account(name='Acme');
insert a;
}
}
}

Use @future to Bypass the Mixed DML Error in a Test Method
Mixed DML operations within a single transaction aren’t allowed. You can’t perform DML on a setup sObject and another sObject in the same transaction. However, you can perform one type of DML as part of an asynchronous job and the others in other asynchronous jobs or in the original transaction. This class contains an @future method to be called by the class in the subsequent example.

public class InsertFutureUser {
@future
public static void insertUser() {
Profile p = [SELECT Id FROM Profile WHERE Name='Standard User'];
UserRole r = [SELECT Id FROM UserRole WHERE Name='COO'];
User futureUser = new User(firstname = 'Future', lastname = 'User',
alias = 'future', defaultgroupnotificationfrequency = 'N',
digestfrequency = 'N', email = 'test@test.org',
emailencodingkey = 'UTF-8', languagelocalekey='en_US',
localesidkey='en_US', profileid = p.Id,
timezonesidkey = 'America/Los_Angeles',
username = 'futureuser@test.org',
userpermissionsmarketinguser = false,
userpermissionsofflineuser = false, userroleid = r.Id);
insert(futureUser);
}
}

This class calls the method in the previous class.

@isTest
public class UserAndContactTest {
public testmethod static void testUserAndContact() {
InsertFutureUser.insertUser();
Contact currentContact = new Contact(
firstName = String.valueOf(System.currentTimeMillis()),
lastName = 'Contact');
insert(currentContact);
}
}

Thanks in Advance,
JThakkar