test: fix incorrect default value

[jh-leong]

In vuejs/core#12227, I attempted to fix the default type restriction in PropOptions, but it caused CI failures (https://github.com/vuejs/ecosystem-ci/actions/runs/11435154446/job/31810009674).

Previously, default allowed arbitrary objects, leading to incorrect tests passing.

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commit: 583c5d7