haskell-ghc-mod: disable test suite to fix build with GHC 7.8.2

https://github.com/kazu-yamamoto/ghc-mod/issues/242
haskell-ghc-mod: make sure GHC <7.8 and >7.8 pick the same datadir install path
pkgs/build-support/cabal: revert "the release version of GHC 7.8.x no longer requires"

This reverts commit a2a398fbda842594ab17d0b98f68dba4c51e49e8. The
issue *does* still exist in GHC 7.8.2. Compiled binaries have no -rpath
into their own install directory ("$out") and thus cannot find their own
shared libraries. To work around this issue, we pass an explicit -rpath
argument at configure time. We do that only on Linux, though, because
-rpath is known to cause trouble on Darwin, which was the reason I
originally reverted that patch.
cabal-bounds: re-generate with cabal2nix
haskell-ghc-mod: re-generate with cabal2nix