TestClass:

@isTest
public class updateContactRole_Test {
    static testMethod void createAccount()
    {    
        Account a = new Account();
        a.Name = 'Test Co.';
        a.BillingStreet = '298 S. Ringo Street';
        a.BillingCity = 'Little Rock';
        a.BillingState = 'AR';
        a.BillingPostalCode = '72201';
        a.BillingCountry = 'USA';
        a.Phone = '501-555-5555';
        a.Website = 'www.testco.com';
        insert a;
        System.debug('created account');
            
        //Then create a primary contact
        Contact c = new Contact();
        c.FirstName = 'Paul';
        c.LastName  = 'Test';
        c.AccountId = a.id;
        c.MailingStreet = '298 S. Ringo Street';
        c.MailingCity = 'Little Rock';
        c.MailingState = 'AR';
        c.MailingPostalCode = '72201'; 
        insert c;
        System.debug('created primary contact');
            
        //Now create an opportunity
        Opportunity o = new Opportunity();
        o.Name = 'New Record';
        o.StageName = 'Posted';
        o.CloseDate = Date.today();
        o.Description = 'Test Record';
        insert o;
        System.debug('created opportunity');
                  
        //Now update the OCR for the primary contact
        OpportunityContactRole ocr = new OpportunityContactRole();
        ocr.ContactId = c.Id;
        ocr.OpportunityId = o.Id;
        ocr.IsPrimary = TRUE;
        ocr.Role = 'Decision Maker';
        insert ocr;
        System.debug('created opportunity contact role for primary');          
        
    }
}

Trigger:

trigger updateContactRole on Opportunity (after insert, after update) {
    
    System.debug('-- Inside Opportunity Contact Role Trigger---');
    
    List<OpportunityContactRole> newContactRoleList = new List<OpportunityContactRole>();
    Set<Id> oppId = new Set<Id>();
    
    for(Opportunity o : [SELECT Id, AccountId FROM Opportunity WHERE AccountId IN: oppId]) {
       
/* Code not covering inside for loop */
 List <Contact> contacts = [SELECT Id FROM Contact WHERE accountId = : o.accountId];
        
        List <OpportunityContactRole> contactRoles = [SELECT Id, ContactId FROM OpportunityContactRole WHERE OpportunityId = :o.Id];
        
        if (contacts.size() > contactRoles.size()) {
            Set<Id> myContacts = new Set<Id>();
            for (OpportunityContactRole contactRole : contactRoles) {
                myContacts.add(contactRole.ContactId);
            }
            for (Contact contact : contacts) {
                if (!myContacts.contains(contact.Id)) {
                    OpportunityContactRole myContactRole = new OpportunityContactRole();
                    myContactRole.ContactId = contact.Id;
                    myContactRole.OpportunityId = o.Id;
                    newContactRoleList.add(myContactRole);
                    
                    // To prevent duplicate.
                    myContacts.add(contact.Id); 
                }
            }
        }
    }
    
    try {
        if(newContactRoleList.size()>0) {
            insert newContactRoleList;
        }
    }

    catch(Exception e) {
        System.debug(e);        
    }
}