Resolve "Add Frontend tests again" NOCHANGELOG

Closes #1716 (closed)

I just reintroduced the tests we already had. I removed the ones which relied on sinon or other bigger dependencies. I think we should wait if vitest supports their functionality or consider each dependency before spending time on making the tests work again. moxios might also be replaced in the future.

My primary focus was to create the structure again to enable devs to provide unit tests for their code.