Make it easier to run Haddock's testsuite

At Zurihac we noticed that it is currently not at all easy to run Haddock's testsuite. Not only can you not use Hadrian but you also seemingly cannot use a released cabal-install. I found it was necessary to build cabal-install from master and use it to build the testsuite with cd utils/haddock; cabal test -w /path/to/ghc after ensuring that Hadrian builds Haddock documentation (e.g. hadrian/build docs).