Running GHC test-suites for our haskell.nix compilers

[angerman]

We should be running GHC test-suites for our GHC compilers too. This will require some re-engineering of being able to independently run the GHC test-suites.

[stale]

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[angerman]

I still think we should do this. I don't know in which form yet though. The best solution still seems to be to just have patched GHCs on GH and build them, as well as their testsuite there.