Hi Team,
I have written a wrapper class contaning the search functionality. But I can't understand how to write it's test class.

APEX CODE 

Public class fetchserials{
    
    Public string OrderID {get; set;}
    Public order OrderInstance {get; set;}
    Public List<OrderItem> OrderItemList {get; set;}
    Public Set<id> ProductIdSet {get; set;} //list to add product Ids
    Public list<Serial_Number__c> SerialList {get; set;}
    Public list<Serial_Number__c> SelectedSerialList {get; set;}
    Public Map<OrderItem,list<Serial_Number__c>> OrderVSerial{get; set;} // list to add OrderItem with their Serial Numbers
    Public List<wrapperfetch> fetchseriallist {get; set;}
    // public Integer Counter;
    Public List<Order_Line_Item_Serials__c> insertordersn {get; set;} // list to add the selected orders
    Public map<id,Serial_Number__c> smap = new map<id,Serial_Number__c>();
    public List<Serial_Number__c> serialNumbers;
    public string fullname{get;set;}
    public string finalSearchValue{get;set;}
    
    //ApexPages.Message myMsg = new ApexPages.Message(ApexPages.Severity.error,'Enter the List');
    // ApexPages.addMessage(myMsg);
    
    Public fetchserials(){
        OrderItemList = new List<OrderItem>(); // intialize
        OrderVSerial = new Map<OrderItem,List<Serial_Number__c>>();
        fetchseriallist = new List<wrapperfetch>();
        insertordersn = new List<Order_Line_Item_Serials__c>();
        SelectedSerialList = new list<Serial_Number__c>();
        SerialList = new list<Serial_Number__c>();
        OrderID = ApexPages.currentPage().getParameters().get('Id'); //Getting the id of current order
      OrderInstance = [SELECT Id FROM Order where Id=:OrderID ];
       OrderItemList=[SELECT Id,OrderId,product2Id,PricebookEntry.Product2.Name, Quantity FROM OrderItem where OrderId =:OrderID];
       loadSerials();
    }
    //adding page reference of the current page
  /*  public PageReference step1() {
        loadSerials();
          if(SerialList==null){
ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.Error, 
'Please select the serial number'));
} 
        return Page.fetchserials;
    }*/
    
    public PageReference cancel() {
        PageReference c=new PageReference('/'+OrderID);
        return c;
    }   
    
    Public void loadSerials(){
        ProductIdSet = new set<Id>();
        For(OrderItem items :OrderItemList){
            ProductIdSet.add(items.Product2Id);// order items products add into productidset 
        }
        SerialList = [SELECT Id,Serial_Number__c,Product_Name__c ,Name,Product__c,Status__c,Bin_Number__c,Location__c
                      FROM Serial_Number__c where Status__c=FALSE And Product__c In:ProductIdSet limit 200 ];
        system.debug('Serialx' +SerialList );
        
        /*if(SerialList == NULL)
{
SerialList.addError('There are no values in the Serials List.'); 
}*/
        // add error exception handling
        system.debug('Serialx' +SerialList );
        For(OrderItem item :OrderItemList){
            //counter = 0;
            For(Serial_Number__c sn :SerialList ){
                If( sn.Product__c == item.Product2Id) // && counter < item.Quantity)
                {   SelectedSerialList = new list<Serial_Number__c>();
                 //     mahesh   fetchseriallist.add(new wrapperfetch(sn,item));
                 fetchseriallist.add(new wrapperfetch(sn));
                 SelectedSerialList.add(sn);
                 //counter = counter +1; 
                }
                else{continue;}
            }
            OrderVSerial.put(item,SelectedSerialList);
            /*=====
try{
if(SerialList == NULL ){ ApexPages.addMessages(e); }
} 
catch (DMLException e){
ApexPages.addMessages(myMsg);
// Code for sending Email
}
============*/
        }
    }
    
    public void search(){
        finalSearchValue = '%' + fullname + '%';
       System.debug(finalSearchValue);
        fetchseriallist = new list<wrapperfetch>();
        List<Serial_Number__c> serialList =  [Select Id, Serial_Number__c, Product_Name__c, Status__c, Bin_Number__c, Location__c FROM Serial_Number__c
                                  where Serial_Number__c like :finalSearchValue];
            System.debug(serialList.size());
        for(Serial_Number__c s : serialList){
                                      System.debug(s);
                                      fetchseriallist.add(new wrapperfetch(s));    
                                      System.debug(s);
                                  }
        
    }
    
    Public class wrapperfetch{
        Public Boolean selected {get; set;}
        Public Serial_Number__c sno{get; set;}
        Public string prodname {get; set;}
        Public OrderItem Oitem{get; set;}
        Public string serialno {get; set;}
        Public Boolean status {get; set;}
        Public String locatn{get;set;}
        Public String binNo{get;set;}
        Public Id stid {get; set;}
        Public Id opid {get; set;}
        
        //    Public wrapperfetch(Serial_Number__c sn,OrderItem Oi){
        Public wrapperfetch(Serial_Number__c sn){  
            //this.selected=check;
            //    this.Oitem= Oi;
            //    this.opid =Oi.Id;
            this.prodname =sn.Product_Name__c;
            this.sno=sn;
            this.locatn=sn.Location__c;
            this.binNo=sn.Bin_Number__c;
            this.serialno = sn.Serial_Number__c;
            this.status=sn.status__c;
            this.stid=sn.id;
            
        }
    }
    
    Public PageReference AddSerailNumbertoOrder(){
        
        for (wrapperfetch st: fetchseriallist ){
            
            If (st.selected == true ){
                Order_Line_Item_Serials__c ordersn= new Order_Line_Item_Serials__c();
                //Order_Line_Item_Serials__c make it to true
                //make a new Order_Line_Item_Serials__c list and make it to true.
                // put that into   ordersn.Serial_Number__c=st.stid;  a new list 
                ordersn.Serial_Number__c=st.stid;
                ordersn.Order_Product__c=st.opid;
                insertordersn.add(ordersn);
                
                Serial_Number__c  serials = new Serial_Number__c();
                serials.id = st.stid;
                serials.Status__c = TRUE;
                serials.Location__c=st.locatn;
                serials.Bin_Number__c = st.binNo;
                SelectedSerialList.add(serials);
                smap.put(serials.id,serials );
                
            }
            
        }
        if (insertordersn.size() > 0){    
                insert insertordersn;    
              //  update smap.values();    
            }
        return Page.fetchserials;    
    }       
}

Please help me to write test class for this code.

Thanks,
Mahesh