[next/head] Add helmet-like .peek() support

[ljosberinn]

Feature request

Is your feature request related to a problem? Please describe.

I'm trying to test a component that does nothing but syncing some state into through next/head. However, next/head is actually a null component that just emits changes internally.
Sadly I haven't found any, not even a hacky, solution to spying on the element creation...

Describe the solution you'd like

A static method like react-helmets .peek() that can be used in tests. which returns the currently existing HTMLCollection just like a document.querySelector('head').children would.

Describe alternatives you've considered

What I've tried:

• creating my own HeadManagerContext.Provider
• spying on HTML.prototype.insertBefore
• spying on initHeadManager return values
• rendering the whole Document / my Custom Document
• cloning & mapping children of those both, spreading my element in there
• rendering to static markup (which works for Document, on a sidenote)

Additional context

This was with jest@latest + @testing-library/react@latest.