test: enable cyrillic punycode test case

The test from RFC 3492 contains a bug: the uppercase D in the input
should be lowercased.  Fix that and enable the test.

Fixes: https://github.com/nodejs/node/issues/8691
PR-URL: https://github.com/nodejs/node/pull/8695


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: James M Snell <jasnell@gmail.com>