TestDeleteRestrictInvoice.cls

@isTest
private class TestDeleteRestrictInvoice {

    // Invoice generator, with or without a Line Item
    static Invoice__c createNewInvoice(Boolean withLineItem) {
        // Create test Invoice and insert it into the database
        Invoice__c invoice = new Invoice__c();
        insert invoice;

        // Create test Line Item and insert it into the database, if withLineItem == true
        if (withLineItem) {
            Line_Item__c item = new Line_Item__c(
                name = '1',
                Quantity__c = 1,
                Invoice__c = invoice.Id
            );
            insert item;
        }        
        return invoice;
    }

    // Single row Invoice with no Line Items => delete
    static testMethod void verifyInvoiceNoLineItemsDelete(){    
        // Create test Invoice and insert it
        Invoice__c invoice = createNewInvoice(false);

        // Delete the Invoice, capture the result
        Database.DeleteResult result = Database.delete(invoice, false);

        // Assert success, because target Invoice doesn't have Line Items
        System.assert(result.isSuccess());   
    }

    // Single row Invoice with Line Items => delete restrict
    static testMethod void verifyInvoiceLineItemsRestrict(){    
        // Create test Invoice and Line Item and insert them
        Invoice__c invoice = createNewInvoice(true);

        // Delete the Invoice, capture the result
        Database.DeleteResult result = Database.delete(invoice, false);

        // Assert failure-not success, because target Invoice has tracks
        System.assert(!result.isSuccess());
    }

    // Bulk delete of Invoice, one without Line Items, another with
    static testMethod void verifyBulkInvoiceDeleteRestrict(){    
        // Create two test Invoices, one with and without a Line Item
        Invoice__c[] invoices = new List<Invoice__c>();
        invoices.add(createNewInvoice(false));
        invoices.add(createNewInvoice(true));

        // Delete the Invoices, opt_allOrNone = false, capture the results.
        Database.DeleteResult[] results = Database.delete(invoices, false);

        // Assert success for first Invoice       
        System.assert(results[0].isSuccess());   
        // Assert not success for second Invoice
        System.assert(!results[1].isSuccess());
    }
}