Fix test section

utf8-string.cabal

@@ -36,4 +36,5 @@ library
 test-suite unit-tests
   type:               exitcode-stdio-1.0
   main-is:            tests/Tests.hs
-  build-depends:      HUnit >= 1.3 && < 1.4
+  build-depends:      base, HUnit >= 1.3 && < 1.4
+  default-language:   Haskell2010