Introduce testing of components

[matus-vacula]

We should fix or remove the current only existing test: src/App.test.jsx
And instead create at least 1 test file for 1 component, to be placed next to it.

the test runner (vitest) will run all js/x/ts files that have .test. in their name (also .spec. but don't use that).
Or would recommend to copy the old src/App.test.jsx tests and start from their with your component of choice

AC:

• we have some tests for at least 1 component
• tests are passing on PRs

https://testing-library.com/docs/react-testing-library/intro/