refactor: migrated the tests to jest

refactor: migrated the tests to jest

* The https://thymikee.github.io/jest-preset-angular/docs[documentation from jest-preset-angular] was slightly outdated
* Therefore, the configuration was copied from the example https://github.com/thymikee/jest-preset-angular/tree/main/examples/example-app-v20[example-app-v20] of the sources (Version `v15.0.2`)
* The configuration of the example was simplified, using the setup `ems-isolated`
* A *Downgrad* was needed for Angular from `20.3.0` to `20.0.0`