trigger HistoryTracking on Case (before update)
{
    Map<string, Schema.SobjectField> caseFields = Schema.SObjectType.Case.fields.getMap();
   
    case caseold=trigger.old[0];
    For (Case cs : Trigger.new)
    {
        for (string fieldName : caseFields.keySet())
        {
            if ( cs.get(fieldName) != Trigger.oldMap.get(cs.id).get(fieldName))
            {
              string oldvalue = string.valueOf(caseold.get(fieldName));
              string newvalue= string.valueOf(cs.get(fieldName));
             
              string parentoldvalue = string.valueOf(caseold.get('ParentId'));
              string parentnewvalue= string.valueOf(cs.get('ParentId'));
        
              if(fieldName != 'Description')
               cs.History_Tracking__c = 'Changed '+fieldName +' From '+oldvalue +' to '+ newvalue;   ------------->this line doen't  cover the code coverage
              else
                 cs.History_Tracking__c = 'Changed On Long Field' ; ------------->this line doen't  cover the code coverage
                
             if(parentoldvalue != null && parentnewvalue== null )
             {
                 cs.CheckParent__c='RemoveParent';   ------------>this line doen't  cover the code coverage
                 system.debug('______________checkparentid+++++++++++++++++++'+cs.CheckParent__c);
             }
             else if( parentnewvalue != null)
             {
                  cs.CheckParent__c='';
             }
                
                 break;
             }  }
    }}
test class
=============
@isTest(seeAllData=true)
private class Testcasehistory
{
    static testMethod void updatefields()
    {
        list<Case> cslist=new list<case>();
        case cs=new case();
        cs.Origin='Web';
        cs.Reason='other';
        cs.Priority='Medium';
        cs.Status='New';
        cs.Type='Other';
        cs.Subject='ESTDeveloper';
        cs.Priority='1 - Critical';
        cs.TFS_WorkItem_Type__c='Bug';
        cs.Description='changedSFDCTOTFS';
        cs.ParentId=cs.id;
       
         // System.assertEquals(cs.Description,' cs.History_Tracking__c');
          // cs.History_Tracking__c='Changed On Long Field';
       
           if(cs.ParentId == null)
           {
           cs.CheckParent__c='RemoveParent';
           }
           else if(cs.ParentId != null)
             {
           cs.CheckParent__c='';
            }
       // cs.Origin='Email';
        cslist.add(cs);
        test.startTest();
        insert cslist;
       
        cs.Origin='Email';
        update cslist;
        test.stopTest();
    }
}