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>