There were some breaking changes over the last few patches so we bump
the interface version. This causes a big problem with testing:

1. To generate cross package docs, we first need to generate docs for
the package used.
2. To generate package docs with new interface version, we need to use
Haddock which has the version bumped.
3. To get Haddock with the version bump, we first need to test cross
package docs
4. GOTO 1

So the problem is the chicken and the egg problem. It seems that the
only solution would be to generate some interface files on the fly but
it is non-trivial.

To run this test, you'll have to:

* build Haddock without the test (make sure everything else passes)
* rebuild the packages used in the test with your shiny new binary
  making sure they are visible to Haddock
* remove the ‘_hidden’ suffix and re-run the tests

Note: because the packages currently used for this test are those
provided by GHC, it's probably non-trivial to just re-build them.
Preferably something less tedious to rebuild should be used and
something that is not subject to change.