Feature: Unit testing

[ayshiff]

I felt that avatar-generator needed some testing.
This PR setup the testing environment using bs-jest/bs-react-testing-library and add some tests here and there.
It's not much, but it's always better to have some tests.

Testing a ReasonReact component

bs-react-testing-library + bs-jest

I created a wrapper component called TestId which pass the data-testid (used by react-testing-library) props to its children.
As Reason doesn’t support using props with dashes.
I don't really like this method as it requires adding extra code to the source files, but it works.

1 - Wrap your component using the TestId Wrapper inside your source file
2 - Render your component inside your test file
3 - You can query your elements using your data-testid or using other queries (queryByLabelText, queryByTitle, queryByText...) and fire some events.

Testing a Reason function

bs-jest

NOTE: I had to add a babel.config.js file and a transformIgnorePatterns options in the jest configuration because of the es6 modules (related issue).
I also had to add the preset babel-preset-gatsby because gatsby returned an error during the compilation if I did not specify it.