I have a requirement to set up a scheduled batch that will run every day & calculate the duration between the last date of opportunity stage changes to the current date.

1. Duration >=25 & Duration <30(Send Email & update opportunity fields)
2. Duration >30 (Update Stage,Other fields)

My code is working as expected but i don't have any clue cover these use cases because OpportunityHistory object is not writeable. Is there any way to handle this use case like setting up mock test or anything.

Batch Class:
 

global class StageAge implements Database.Batchable<sObject>  
{
    public static List<Messaging.SingleEmailMessage> mails = new List<Messaging.SingleEmailMessage>();
    public static id owea = [select Id,address from OrgWideEmailAddress where Address = 'info@a.com' Limit 1].id;
    public Static Id Inboundtemplate = [select id from EmailTemplate where name ='Handle CloseDate' Limit 1].id;
    global Database.QueryLocator start(Database.BatchableContext bc)
    {
       return Database.getQueryLocator('Select id from opportunity where isclosed=false AND Team__c=\'Bangalore Team\''+
                                      'AND StageName != \'Nurture\''); 
    }
    global void execute(Database.BatchableContext bc, List<Opportunity> OpportunityList)
    {
        Map<Id,Opportunity> UserDetails = new Map<Id,Opportunity>();
        If(OpportunityList <> null && !OpportunityList.isEmpty())
        {
            for(opportunity opps:[Select id,ownerid,name,Owner.FirstName,Owner.LastName from opportunity where id in:OpportunityList])
            {
                UserDetails.put(opps.id,opps);
            }
        }
        System.debug('Details Opportunity'+OpportunityList.size());
        Map<Id,Integer> Opportunityids = new Map<Id,Integer>();
        List<Opportunity> Nurtureupdate = new List<Opportunity>();
        Map <Id,OpportunityHistory> LastStage =new Map <Id,OpportunityHistory>();
        For(OpportunityHistory opps:[Select id,OpportunityId,CreatedDate from OpportunityHistory
                                     where OpportunityId IN:OpportunityList ORDER BY CreatedDate DESC NULLS First])
        {
            If(!LastStage.containsKey(opps.OpportunityId))
            {
                System.debug('Inside Loop'+opps.OpportunityId);
                System.debug('Create date '+opps.CreatedDate);
                LastStage.put(opps.OpportunityId,opps);  
                Integer Duration=0;
                //Datetime MainCreateddate = opps.CreatedDate;
                Duration=opps.CreatedDate.Date().daysBetween(Date.today());
                System.debug('Duration Details'+Duration);
                If(Duration >= 25 && Duration < 30)
                {
                    System.debug('Inside Duration');
                    EmailDispatcher(opps.OpportunityId,UserDetails.get(opps.OpportunityId).ownerid,UserDetails.get(opps.OpportunityId).name
                                   ,UserDetails.get(opps.OpportunityId).Owner.FirstName,UserDetails.get(opps.OpportunityId).Owner.Lastname);
                    Opportunityids.put(opps.OpportunityId,Duration);
                }
                If(Duration >30)
                {
                    Opportunityids.put(opps.OpportunityId,Duration);
                }
            }          
        }
        if(mails!=null && !mails.isEmpty())
        {
            System.debug('Its working');
            Messaging.sendEmail(mails);
        }
        If(Opportunityids <> null && !Opportunityids.isEmpty())
        {
            For(Opportunity oppupdate:[Select id,stagename from opportunity where id in:Opportunityids.keyset()])
            {
                System.debug('Opportunity_Duration_Before_Check'+Opportunityids.get(oppupdate.id));
                If(Opportunityids.get(oppupdate.id) >30)
                {
                    Opportunity Oppaddition     = new Opportunity();
                    Oppaddition.id              = oppupdate.id;
                    Oppaddition.stagename       = 'Nurture';
                    System.debug('******Opportunity>30'+Opportunityids.get(oppupdate.id));
                    Oppaddition.Stage_Age__c    = Opportunityids.get(oppupdate.id); 
                    Nurtureupdate.add(oppaddition);
                }
                System.debug('**********Opportunity_Before_Check<30'+Opportunityids.get(oppupdate.id));
                If(Opportunityids.get(oppupdate.id) >= 25 && Opportunityids.get(oppupdate.id) < 30)
                {
                    Opportunity Oppaddition     = new Opportunity();
                    Oppaddition.id              = oppupdate.id; 
                    System.debug('**********Opportunity<30'+Opportunityids.get(oppupdate.id));
                    Oppaddition.Stage_Age__c    = Opportunityids.get(oppupdate.id); 
                    Nurtureupdate.add(oppaddition);
                }                
            }
            If(Nurtureupdate <> null && !Nurtureupdate.isEmpty())
            {
                update Nurtureupdate;
            }
        }
    }
    Public Static void EmailDispatcher(Id OpportunityDetails,Id Target,String Opportunityname,String Firstname,String Lastname)
    {
        System.debug('Target details'+Target);
        System.debug('Inside Dispatcher');
        System.debug('OpportunityDetails'+OpportunityDetails);
        Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
        //mail.settemplateid(Inboundtemplate);
        mail.setTargetObjectId(Target);
        mail.setTreatTargetObjectAsRecipient(true);
        //mail.setToAddresses(new String[] {'toml@a.com'});
        mail.setOrgWideEmailAddressId(owea);
        mail.setUseSignature(false);
        mail.setSaveAsActivity(false);
        mail.setReplyTo('marketing@a.com');
        mail.setsubject('Update CloseDate of Opportunity Name '+Opportunityname);
        String body = 'Hi '+Firstname+' '+Lastname+','+'<br/>'+'<br/>';
        body += 'Opportunity Name '+Opportunityname+'<br/>';
        body += 'Have we closed this one? Please update the Close date accordingly.'+'<br/>';
        body += 'For More details'+'<a href="'+OpportunityDetails+'">'+'Click Here'+'</a>'+'<br/>'+'<br/>';
        body += 'Thanks you again'+'<br/>';
        body += 'Team Azuga'+'<br/>'+'<br/>';
        mail.setHtmlBody(body);
        mails.add(mail);
    }
    global void finish(Database.BatchableContext bc)
    {
        
    }
}