Account a = [select name from Account where Id='0010Q00000DO8b7QAD'];

Contact c = New Contact(LastName = 'Test', AccountID = a.id);
insert c;

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

    );

system.debug('User = ' + u.Id);

insert u;

return u;