I have developed a test class which automatically converts opportunity products to assets taking dependant values as seen below:
 

trigger OpportunityAssetonClosedWon on Opportunity (after insert, after update) {
    for(Opportunity o: trigger.new){
        if(o.isWon==true && o.HasOpportunityLineItem==true){
            String opptyId = o.Id;
            OpportunityLineItem[] OLI = [Select UnitPrice, Quantity, PricebookEntry.Product2Id, Product2.Name,
                                         Product2.Family, PricebookEntry.Product2.Name
                                         From OpportunityLineItem 
                                         WHERE OpportunityId=:opptyId];
            Asset[] ast = new Asset[]{};
            Asset a = new Asset();
            for(OpportunityLineItem ol: OLI){
            if(ol.Product2.Family=='Terminal' || ol.Product2.Family=='Gateway' ){
                a = new Asset();
                a.AccountId = o.AccountId;
                a.Product2Id= ol.PricebookEntry.Product2Id;
                a.Quantity= 1;
                a.Price= ol.UnitPrice;
                a.PurchaseDate=o.CloseDate;
                a.status='Purchased';
                a.Name = ol.Product2.Name;
                ast.add(a);
                ol.Converted_to_Asset__c = true;
            }
            }
                update OLI;
                insert ast;
            }
        }
    }

I am having trouble structuring my test class but ive finally worked out my errors. I think I am missing something. Test class is shown below

 

@isTest
public class OpportunityAssetonClosedWonTest {
	static testMethod void closedOpportunity() {
        Account testAcc = new Account(
        Name = 'TestAccount'
        );
        insert testAcc;
        
        Pricebook2 p = new Pricebook2(
        	Name = 'Testbook');
        insert p;
        
		Opportunity testOpportunity = new Opportunity(
            StageName = 'Sourcing Demand',
            CloseDate = Date.newInstance(2017,12,31),
            AccountId = testAcc.Id,
            Name = 'Test Opportunity Triggers',
            Pricebook2Id=p.id
        );
        insert testOpportunity;
        
        Product2 p1 = new Product2(
        	Name = 'prod1',
        	Family = 'Terminal');
        insert p1;
        PricebookEntry pE = new PricebookEntry(
        	Pricebook2Id=p.Id,
        	Product2Id=p1.Id);
        Product2 p2 = new Product2(
        	Name='prod2',
        	Family='Gateway');
        insert p2;
        PricebookEntry pE2 = new PricebookEntry(
        	Pricebook2Id=p.Id,
        	Product2Id=p2.Id);
        Product2 p3 = new Product2(
        	Name = 'prod3',
            Family = 'Card Not Present');
        insert p3;
        PricebookEntry pE3 = new PricebookEntry(
        	Pricebook2Id=p.Id,
        	Product2Id=p3.Id);
        
        
        OpportunityLineItem OLI1 = new OpportunityLineItem(
            OpportunityId=testOpportunity.Id,
            Quantity = 1,
            UnitPrice = 1
        );
        insert OLI1;
        
        testOpportunity.StageName = 'Closed/Won';
        
        Test.startTest();
        
        update testOpportunity;
        
        Test.stopTest(); 
	}
}