Avoid errors on non UTF-8 Windows

Created by: matsubara0507

about #135 (closed) but not file path, error is occur if the file contents includes multibyte strings (e.g. Japanese).

This problem like haddoc issue https://github.com/haskell/haddock/pull/566 .

Environment

OS: Windows 10 tasty-discover: 4.1.3 GHC: 8.2.2

> stack test
tasty-discover-4.1.3: build (lib + exe + test)
Preprocessing library for tasty-discover-4.1.3..
Building library for tasty-discover-4.1.3..
Preprocessing executable 'tasty-discover' for tasty-discover-4.1.3..
Building executable 'tasty-discover' for tasty-discover-4.1.3..
Preprocessing test suite 'test' for tasty-discover-4.1.3..
Building test suite 'test' for tasty-discover-4.1.3..
tasty-discover: test\ConfigTest.hs: hGetContents: invalid argument (invalid byte sequence)
`tasty-discover' failed in phase `Haskell pre-processor'. (Exit code: 1)
Progress: 1/2
--  While building custom Setup.hs for package tasty-discover-4.1.3 using:
      C:\Users\Hoge\AppData\Roaming\stack\setup-exe-cache\i386-windows\Cabal-simple_Z6RU0evB_2.0.1.0_ghc-8.2.2.exe --builddir=.stack-work\dist\010ee936 build lib:tasty-discover exe:tasty-discover test:test --ghc-options " -ddump-hi -ddump-to-file -fdiagnostics-color=always"
    Process exited with code: ExitFailure 1

I fixed this problem, but I have not idea how to test this case. Because, tasty-discover's test use tasty-discover .