Expose AmpStateContext for testing purpose

[amiralies]

Feature request

I've custom higher level amp safe components that behave differently on amp / react version of app.
The problems is that for testing those components I've to import AmpStateContext from next/dist/next-server/lib/amp-context.

Describe the solution you'd like

I like to import it from next/amp or next/amp-context. though it should be documented that's for testing purpose.

Describe alternatives you've considered

importing AmpStateContext from next/dist/next-server/lib/amp-context. Mocking can be an option too.