chore: update PostCSS and tests

[baseballyama]

I created this PR because eco-system-ci is failed.
https://github.com/sveltejs/svelte-ecosystem-ci/actions/runs/5483027087/jobs/9994935125

After investigation, I found that this was due to a change in the order of JSON properties when postcss 8.4.25 was released.
I think we can not say that this was not a bug, so I simply upgraded the postcss version and fixed tests.

[changeset-bot]

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Latest commit: 1b29541

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[baseballyama]

I think we don't need a changeset for this?

[Conduitry]

How are the actual results being compared with these JSON files? The better solution seems to me to be to make that comparison able to deal with keys in different orders.

[ota-meshi]

Thank you!
Ordering issues may cause problems again, but I think the changes in this PR are fine now so I'm merging this.

[baseballyama]

Currently, JSON is mainly compared as a string. I implemented the process of sorting JSON properties before create this PR, but since it has some performance impact and situations like this do not occur often, So I simply updated tests.
If this happens again, I will add this process and then compare them as JSON.