…ff of it

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Closes #791 



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Partially Closes #791 

This also refactors the test infrastructure to use absolute URLs instead
of relative URL (closes #643 )

### How to test the Change
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- Start up the wp-env
- `npm run build -- --filter=@10up/wp-nextjs-app` to build the app
- `npm run start -w=@10up/wp-nextjs-app` to start the app.
- Make sure to set the headless front-end url in Settings.
- Open any post on the admin interface and update it, ensure that the
post on the front-end gets revalidated (yuou see the updated version)


### Checklist:
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- [x] All new and existing tests pass.