Hello guix,

I manage to fix all test failures of haskell-mode (which FTBFS for a
long time). I use Jack's bug report
<https://github.com/haskell/haskell-mode/issues/1601> as a starting
point and continue from there. The fix consists of backporting 2 commits
from upstream and hard-coding the filename in the tests. (This is okay
since the tests will not be installed.) 

Let's write some Haskell for good (`･ω･´)