More tests for Authentication app

Please add tests for:

- [ ] MSISDN based authentication using the [custom auth backend](https://github.com/praekelt/ghr-mobi/blob/96d84992e0dccd5bf360d98124a9092840b2bcea/src/app/authentication/backends.py#L12)
- [ ] the random image assignment if [none is specified](https://github.com/praekelt/ghr-mobi/blob/96d84992e0dccd5bf360d98124a9092840b2bcea/src/app/authentication/models.py#L110).