Disable tests in Elm

Elm does not serve any tests besides a test that always fails. Additionally, even calling this fails since no executable is created.

The relevant lines from the build log are:

> Building Elm-0.9.0.1...
> Preprocessing test suite 'test-elm' for Elm-0.9.0.1...
> [1 of 1] Compiling Everything       ( tests/Everything.hs, dist/build/test-elm/test-elm-tmp/Everything.o )
> Warning: output was redirected with -o, but no output will be generated
> because there is no Main module.

> running tests
> Running 1 test suites...
> Setup: Error: Could not find test program "dist/build/test-elm/test-elm". Did
> you build the package first?
> builder for `/nix/store/m01c1jnyi5098fb1v7ag5pjv6y0nmrpy-haskell-Elm-ghc7.6.3-0.9.0.1.drv' failed with exit code 1
> error: build of `/nix/store/m01c1jnyi5098fb1v7ag5pjv6y0nmrpy-haskell-Elm-ghc7.6.3-0.9.0.1.drv' failed