Testsuite infrastructure is too complex

In !2465 (closed) we had some trouble finding the root cause of a test failure and accepting the test result didn't work. It could probably be fixed by passing appropriate parameters or something, but it would be better to fix the root cause: our test infrastructure is too complex.

Example of test execution sequence (the failing one in !2465 (closed)):

Hadrian
  -> Make (test driver)
    -> Python (all.T)
      -> Make (testsuite/test/haddock/haddock_testsuite/Makefile)
        -> GHC: build `htmlTest` program (test driver from Haddock submodule)
        -> htmlTest
          -> call Haddock program
          -> accept or not the result

I think we should get rid of the "Make -> Python (and external tools like grep/sed/awk) -> Make" steps that bring complexity without additional value. It would be great to have an Haskell testsuite driver. While #17527 (closed) is not fixed, it would have to be usable by the legacy build system too.