Re-enable tests

As part of #689 I deactivated a couple of tests, since the test-setup with fixtures had cyclic dependencies. This needs to be reworked as part of this ticket and make tests available again through vitest. Somewhen during development - I thought - it was possible to run vitest wo/ having the code `build` before. As part of this ticket, investigate, if bringing back `vite` to each ticket (with its config), it can run tests wo/ them being built by `tsup`.