CRM-12845 - CRM_Utils_Migrate_Export - Add test

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* CRM-12845: CRM_Utils_Migrate - Handle CustomGroup subtypes
  http://issues.civicrm.org/jira/browse/CRM-12845
CRM-12845 - CRM_Utils_Migrate_Import - Add test

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* CRM-12845: CRM_Utils_Migrate - Handle CustomGroup subtypes
  http://issues.civicrm.org/jira/browse/CRM-12845
CRM-12845 - Change name of test-class to reflect dual purpose

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* CRM-12845: CRM_Utils_Migrate - Handle CustomGroup subtypes
  http://issues.civicrm.org/jira/browse/CRM-12845