Add JEST unit test

[MelleD]

Summary:
Implement comprehensive unit tests for the expander components using the Jest testing framework to ensure code quality and functionality.

Description:
To maintain high code quality and ensure the functionality of the expander components, we propose adding unit tests using Jest. This will help in identifying bugs early, verifying component behavior, and making future code changes more reliable.

npm install --save-dev jest
npm install --save-dev @testing-library/jest-dom
npm install --save-dev @testing-library/svelte