ignore packaging test fixtures

[Rich-Harris]

no idea why this didn't show up sooner, but git clean -xdf && pnpm i && pnpm build creates a bunch of untracked files that should be ignored

[changeset-bot]

⚠️ No Changeset found

Latest commit: a49b5ee

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[Rich-Harris]

actually that's not true, it was only happening on #4710 because i borked up the rimraf implementation. carry on