cryfs: disable tests on Darwin

The tests [require macFUSE to be installed][1] on Darwin. This change
disable the tests because Hydra doesn't have it installed.

[1]: https://github.com/NixOS/nixpkgs/blob/master/doc/builders/packages/fuse.section.md

pkgs/tools/filesystems/cryfs/default.nix

···
     "-DBUILD_TESTING:BOOL=${if doCheck then "TRUE" else "FALSE"}"
   ] ++ lib.optional doCheck "-DCMAKE_PREFIX_PATH=${gtest.dev}/lib/cmake";
 
-  doCheck = true;
+  # macFUSE needs to be installed for the test to succeed on Darwin
+  doCheck = !stdenv.isDarwin;
 
   checkPhase = ''
     # Skip CMakeFiles directory and tests depending on fuse (does not work well with sandboxing)