next/font: Align with Next.js integration test suite (#4039)
This:
* Adds a compile-time feature, `__internal_nextjs_integration_test`,
which is set when building Turbopack for the Next.js integration test
suite.
* When built with `__internal_nextjs_integration_test`, expects the
environment variable `NEXT_FONT_GOOGLE_MOCKED_RESPONSES` to be set with
an absolute path to a JavaScript file exporting a mapping of urls to
stylesheet text. This is used in place of making real requests to Google
Fonts.
* Addresses some small bugs in next/font/google to bring the
implementation in line with the urls generated by Next.js without
Turbopack.
* Changes classnames generated by next/font to use 6 character hashes,
bringing it in alignment with Next.js without Turbopack.
Test Plan:
* Adjusted existing unit tests
* In Next.js, `node run-tests.js -c 1 test/e2e/next-font/index.test.ts`
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