fix: run onDestroy cleanup during SSR

[Rich-Harris]

Fixes #10296

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[dummdidumm]

Dusted this off, implemented the fix and wondered why it still fails - turns out we're running the SSR tests in DOM mode, too. I had to add a separate vitest config to run the tests in a "real" server environment - couldn't find a better way to achieve this. Makes running the tests a bit more tedious and slower which is a shame, and given that they all run just fine in both environments we could scope this "real SSR environment" to a few specific tests only (for example only run the browser environemtn ssr tests in it). Open to suggestions.