testsuite: Print haddock stdout/stderr on failure

[mpickering]

This greatly helps when there are problems running haddock tests on GHC's CI because otherwise we get the very unhelpful error message:

Failed to run Haddock on test package ''

Now instead we also get the actual problem in the log:

haddock: internal error: /private/var/lib/gitlab-runner/builds/7KQsmDei/0/ghc/ghc/_build/install/lib/ghc-9.7.20230404/lib/../../../share/doc/ghc-9.7.20230404/html/libraries/array-0.5.5.0-inplace/array-0.5.5.0-inplace.conf.haddock: withBinaryFile: does not exist (No such file or directory)