struct Contact {
   //holds whichever properties you need to create a CNContact
}

protocol ContactsHolder {
    func save(contact: Contact)
    func add(contact: Contact)
    func delete(contact: Contact)
    func update(contact: Contact)
    //Maybe more methods, the important thing is that you abstract yourself away from CNContactStore and other Contact kit classes
}

enum ContactsUpdateMethod {
    case save(Contact)
    case add(Contact)
    case delete(Contact)
    case update(Contact)
}

protocol ContactsHolder {
    func execute(_ method: ContactsUpdateMethod)
}

class CNContactsHolder: ContactsHolder {
    func save(contact: Contact) {
        //1. create a `CNContact` from your `Contact`
        //2. create a saveRequest
        //3. execute: CNContactStore().execute(saveRequest)
    }

    ....
}

let contactsHolder: ContactsHolder

init(contactsHolder: ContactsHolder = CNContactsHolder()) {
    self.contactsHolder = contactsHolder
}

let contactsHolder: ContactsHolder

var contactsHolder: ContactsHolder = CNContactsHolder()

struct MockContactsHolder: ContactsHolder {
    var saveWasCalled = false
    func save(contact: Contact) {
        saveWasCalled = true
    }
}