Snapshot tests for a generated stylesheet

A token pipeline whose output is a stylesheet, and no test at all because the transforms had unit tests.

test('generated tokens match the snapshot', async () => {
  const css = await build(tokens)

  await expect(css).toMatchFileSnapshot('./__snapshots__/tokens.css')
})

// which caught: a transform reordering custom properties
// after a dependency bump, so a var() referenced one
// declared below it.

Unit tests on each transform proved every stage correct and said nothing about the composition, which is where the bug was. A snapshot of the final artefact is a weak assertion and it is the only one that covers the pipeline as a whole — and unlike most snapshot tests, this one is on a file that is genuinely generated and genuinely reviewed when it changes.