Create User in Apex Test Class

INTERNAL USER

Profile p = [SELECT Id FROM Profile WHERE Name='Standard User']; 
List<User> uu = new List<User>();
while (uu.size() < 5) {
      Blob b = Crypto.GenerateAESKey(128);
      String h = EncodingUtil.ConvertTohex(b);
      String uid = h.SubString(0,8);
      User u = new User(Alias = uid, Email= uid + '@testdomain.com', 
          EmailEncodingKey='UTF-8', LastName='Testing', LanguageLocaleKey='en_US', 
          LocaleSidKey='en_US', ProfileId = p.Id, 
          TimeZoneSidKey='America/New_York', UserName= uid + '@testdomain.com');      
      uu.add(u);
    }
    insert uu;

EXTERNAL USER OR COMMUNITY USER

Account a = new Account(Name = 'ABC Corp'); // add more fields based on your org validation 
insert a;
Contact c = New Contact(LastName = 'Test', AccountID = a.id);
insert c;
// system.debug('Contact = ' + c.Id);
Profile p = [SELECT Id FROM Profile WHERE Name = 'Customer Community user' LIMIT 1];
user u = New User(
            UserName = 'test_' + math.random() + '@test.com',
            FirstName = 'Test-First',
            LastName = 'Test-Last',
            Alias = 'test',
            email = 'test' + math.random() + '@test.com',
            CommunityNickName = string.valueOf(math.random()).substring(0,6),
            ProfileID = p.id,
            TimeZoneSidKey = 'America/New_York', 
            LocaleSidKey = 'en_US', 
            EmailEncodingKey = 'UTF-8', 
            LanguageLocaleKey = 'en_US',
            ContactId = c.Id
  );
insert u;