Closes WEB-1518
### What?
This PR refactors test cases from next-dev, tests basic tailwind rendering with turbopack. It still runs against plain next.js too. While checking, noticed there's a stub fixture (css-fixture/with-tailwindcss) but those fixture seems not being used in the test. Moved those and utilized. Since this test runs against next.js / turbopack both anyway, moving fixture should not be a huge regression I believe.
