There are 2 ways in which you can set system protected fields for test code coverage:

a) Using Test.loadData()
b) Using JSON.deserialize()
 
1. Using Test.loadData
 
In this option you will need to create a .csv file and enter the necessary data in the .csv file. Once the csv file is ready, upload the csv file as a static resource. 
[Attaching a sample .csv file with the article to try the below example] 
 
a. Click Develop | Static Resources, and then New Static Resource. 
b. Name your static resource testCases. 
c. Choose the file you just created. 
d. Click Save. 
 
Then Call Test.loadData in a test method to populate the test case data. 
 
Code Example: 
@isTest 
private class caseUtil{ 
static testmethod void testLoadData(){ 
List<sObject> ls = Test.loadData(Case.sObjectType,'testCases'); 
 
Case c = (Case)ls[0]; 
 
System.assert(ls.size() == 1); 
String cStatus = c.Status; 
DateTime cDate = c.CreatedDate; 
 
System.debug('Case Id: ' + c.Id); 
System.debug('Case Status: ' + cStatus); 
System.debug('Case Date: ' + cDate); 
 
c.status = 'New'; 
 
update c; 
 
System.debug('Case status: ' + c.status); 
} 
 
2. Using JSON.deserialize 
You can create sObjects in memory with arbitrary CreatedDate values by using JSON.deserialize. This doesn't enforce the normal read-only field attributes that prevent you from setting a createdDate value. 
 
Code Example: 
 
@isTest 
private class CaseTest{ 
static testmethod void testLoadData(){ 
String caseJSON = '{"attributes":{"type":"CasSe","url":"/services/data/v25.0/sobjects/Case/500E0000002nH2fIAE"},"Id":"500E0000002nH2fIAE","Status":"Open","CreatedDate":"2012-10-04T17:54:26.000+0000"}'; 
Case c = (Case) JSON.deserialize(caseJSON, Case.class ); 
System.debug('Test case:' + c.createdDate); 
System.debug('Test caseId:' + c.Id); 
System.debug('Test caseStatus:' + c.status); 
 
Case c1 = new Case(); 
c1.Id = c.Id; 
c1.status = 'New'; 
update c1; 
 
System.debug('Test caseStatus1:' + c1.status); 
 
} 
}