Tests for IDNA2008 (libidn2)

The following discussion from !56 (closed) should be addressed:

• @ondrej started a discussion:

  This mostly looks fine to me (after the nits being fixed), but the biggest issue before merging the code would be having tests for the code.

  Could you come up with couple of tests cases (IDNA2008 compliant, and also some tests that should "fail" because the encoding is broken, etc.) and either write the tests yourself (preferred :)), or just shove it to us and we'll take care of the tests. But they need to be present before the final merge.