Update unit tests to use Jest

We use Jest for our unit tests in most other libraries. We've been migrating all repository test suites to use Jest for consistency.

Start by locating the repository's existing unit-test suites and comparing their current runner setup with the Jest-based approach described in the issue. Identify all suites that need migration and verify that they run successfully under Jest; done means the repository test suites consistently use Jest.

Written by the indexing model from the issue text.