Integration tests for next cache tag revalidations

[kahlstrm]

Currently the code for revalidating pages and globals is quite "hacked together" and barely working. Writing integration tests for these should be a high priority as this is arguably one of biggest key features of this entire thing really.
Integration tests would have to be done for every collection and would be of sort:

1. Render page with collection/global item in it on next.js side, verify the content.
2. Make a change in backend to that specific collection/global
3. Re-render the page and verify that the change has occurred.