Creating unit tests without interacting with the database - creating fake IDs

Here's the fake ID test utility method as used in my DF13 "Tests and Testability" session:

public class TestUtility
{
   static Integer s_num = 1;

   public static String getFakeId(Schema.SObjectType sot)
   {
      String result = String.valueOf(s_num++);
      return sot.getDescribe().getKeyPrefix() + 
         '0'.repeat(12-result.length()) + result;
   }
}

Originally posted here: http://foobarforce.com/2013/08/15/apex-method-of-the-day-string-repeat/

And here is a use of it:

Account a = new Account(Id=TestUtility.getFakeId(Account.SObjectType));
Opportunity o = new Opportunity(AccountId=a.Id);