chore: await toMatchFileSnapshot()

[adiguba]

When running tests, I got warnings like that on the preprocess tests :

Promise returned by expect(actual).toMatchFileSnapshot(expected) was not awaited. Vitest currently auto-awaits hanging assertions at the end of the test, but this will cause the test to fail in Vitest 3. Please remember to await the assertion.
at C:/projects/svelte/packages/svelte/tests/preprocess/test.ts:28:22

So I just added the await to toMatchFileSnapshot()

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pnpm add https://pkg.pr.new/svelte@15303

[adiguba]

Oups I just saw #15301