Test/QuickCheck/All.hs: fix 'quickCheckAll' to read haskell sources in UTF-8

Today I've got an Agda build failure on a box running LANG=C locale (UTF-8
incapable).

    examples/Heap.hs:151:10:
       Exception when trying to run compile-time code:
         examples/Heap.hs: hGetContents: invalid argument (invalid byte
sequence)
         Code: quickCheckAll
       In the expression: $quickCheckAll
       In an equation for `main': main = $quickCheckAll

It's a common error of a 'readFile' assuming locale input. Fixed to read in
UTF-8 in recent GHCs.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>