Hadrian: use the testsuite driver's config.haddock arg more correctly

4 haddock tests assume that .haddock files have been produced, by using the 'req_haddock' modifier. The testsuite driver assumes that this condition is satisfied if 'config.haddock' is non-empty, but before this patch Hadrian was always passing the path to where the haddock executable should be, regardless of whether it is actually there or not.

Instead, we now pass an empty config.haddock when we can't find /docs/html/libraries/array/array.haddock>, and pass the path to haddock when that file exists. This has the (desired) effect of skipping the 4 tests (marked as 'missing library') when the docs haven't been built, and running the haddock tests when they have.

This fixes all remaining test failures for me (building and running tests in a ghc.nix environment), but I also added a command that runs the 'test' rule in the hadrian/linux CI job, to see how we're doing there.