Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/345378

Part of our prettier migration; changing the arrow-parens style.

When using an IDE or an editor, those expect jest.config.js in the
project's root if nothing else is specificed in the config. In case of
two separate configs, we would need to update the configs for every jest
spec run that is not very enjoyable.

**Why?**
To write [FE integration tests][1], we will need
to have two related but different environments.
The best way to do this is with two configs and
a base config.

[1]: https://docs.gitlab.com/ee/development/testing_guide/testing_levels.html#frontend-integration-tests