Apex method to generate fake record ids in tests.

TestUtility.cls

/**
* Apex method to generate fake record ids in tests
* Created by Stephen Willcock
* https://foobarforce.com/2013/08/15/apex-method-of-the-day-string-repeat/
*/
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;
  }
}