We insert the content returning from `useServerInsertedHTML` to head
element for app dir, but it shouldn't only inject once since there're
suspense boundaries that could hold insertions.
## Bug
- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have a helpful link attached, see `contributing.md`
Co-authored-by: JJ Kasper <jj@jjsweb.site>
Jiachi Liu