Create module for app testing

Parent issue #71

## Need

```gherkin
In order to easily add tests when implementing a feature or solving a bug
As a developer
I want to know where to add new tests
```

## Deliverables
- [x] A documented way in which tests are added to the project

## Solution
- [x] Move current tests to a tests module
- [x] Fix failing tests while at it
- [x] Create files for model tests
- [x] Add a testing.md doc file in which the testing routine is explained