Add tests

[knutkirkhorn]

Some links to look at:

• https://github.com/vadimdemedes/ink#testing
• https://github.com/vadimdemedes/ink-testing-library
• https://github.com/vadimdemedes/ink-select-input/blob/master/test.js
• https://github.com/vadimdemedes/ink-text-input/blob/master/test.js
• https://github.com/mordv/mnswpr/blob/master/src/test.tsx
• https://github.com/checkupjs/checkup/tree/master/packages/checkup-formatter-pretty/__tests__