Add watch test ensure to ensure files loaded via `load` hook is not watched

I think we should add tests that ensures:

- ids loaded via `load` hook is not watched
- ids loaded by the file read is watched
- ids added via `addWatchFile` is watched

_Originally posted by @sapphi-red in https://github.com/rolldown/rolldown/pull/6037#pullrequestreview-3194744000_
            