create email notification tests for existing notifications

- [ ] Course Complete
- [ ] Course Track Complete
- [ ] Enrollment
- [ ] Gateway: Manual - Payment Due
- [ ] Lesson Complete
- [ ] Payment Retry Scheduled
- [x] Purchase Receipt
- [x] Quiz Failed - has pr #676
- [x] Quiz Passed - has pr #672
- [ ] Section Complete
- [ ] Student Welcome
- [ ] Subscription Cancellation Notice

Notification controllers should implement new`get_test_settings()` method: https://github.com/gocodebox/lifterlms/blob/master/includes/abstracts/llms.abstract.notification.controller.php#L319-L328

As well as new `send_test()` method: https://github.com/gocodebox/lifterlms/blob/master/includes/abstracts/llms.abstract.notification.controller.php#L445-L455

Existing controllers should implement a method which queries existing actions which triggered the notification in order to provide a list of examples that can be used to send a test email.

This means that there must be sample data on the site which can be used to send the test. EG: a course completion test cannot be sent if there haven't been any course completions.

Example implementation example to come