Fix a bad data type in Hcheck.hs

While trying to understand why some code was not being tested, I
realised that we have a bad data type in Hcheck.hs.

We have "data Level = GroupLvl | ClusterLvl", but then we need to pass
the group name/index as well, so we have functions that look like the
following:

  f :: Level -> Maybe String -> …
  f GroupLvl (Just gname) = …
  f GroupLvl Nothing = … -- never used
  f ClusterLvl _ = … -- wrong, since a (Just x) could be passed

It's clear that we just need to move the group name in the "GroupLvl"
constructor, and simplify the call chain.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>