next/font tests: support Turbopack css module format (#46658)
Turbopack uses a different format for its css module classnames [0].
This adds and uses a helper function to the next-font tests to assert
against these classnames when testing with Turbopack. Ideally these
tests could be more behavioral (e.g. asserting on rendered dimensions),
but these can't capture things like fallback fonts.
[0] https://github.com/vercel/turbo/pull/3437
