Use ExUnit.Case.register_test/6 instead of ExUnit.Case.register_test/…

[nickurban]

Use ExUnit.Case.register_test/6 instead of ExUnit.Case.register_test/4 which is deprecated.

Thanks to @dmitry-semenov for #90 which first included this fix.

@arfl -- is this good to merge?

[axelrizo]

That helped me to remove annoying warnings in elixir 1.18, when executing tests. Thanks!

I'm importing this as:
{:cabbage, git: "https://github.com/nickurban/cabbage.git", ref: "71d3496", only: :test}

Until they decide to merge the commit.