Cleaning up test output

Silences Mix console output during tests.

Hopefully those assert changes are alright, the previous `:ok` outcome seems to just come from `IO.puts`: https://github.com/elixir-lang/elixir/blob/ef23a3fe395059f0d835d9fef8c3bc4dab5ccb68/lib/mix/lib/mix/shell/io.ex#L32

I originally took a different approach, but the elixir-core mailing list set me straight. https://groups.google.com/forum/#!topic/elixir-lang-core/tRkmXu2DfGg