Add more unit and integration tests

[stevenewald]

Now that the testing framework, libraries, and CI workflows are implemented, CRACO could benefit from a comprehensive suite of tests.

Some ideas:

• [x] Postcss config integration test
• [ ] Babel config integration test
• [ ] CRACO plugin integration test
• [x] Webpack config integration test
• [ ] Fix 1-3 posted issues and create unit tests