Improvements to the test suite

[aleclarson]

The old test suite broke a while back when we migrated from tsconfig-paths to tsconfck. It used Jest. I just removed that test suite, since it wasn't doing us any good. I also didn't like its approach, which involved a lot of module mocking.

If someone wants to contribute a new test suite, here are the guidelines:
1. Use Vitest
2. Focus on E2E integration tests. To accomplish this, you could add a spec/__fixtures__ folder with various subfolders (one per test case, to keep things separated).
3. Try covering all test cases that were previously covered (see here). If you don't have the bandwidth, that's fine. Maybe someone else can help out. Even if you can only provide the most basic integration test, it would be appreciated.
4. Restore the test script in package.json
5. Re-enable the GitHub workflow (see here). You can keep the Codecov upload (its secret is still intact).

[aleclarson]

Vitest has been set up in 1cfbe4d.

• Re-enable the GitHub workflow
• Add more test cases