[v6.d REVIEW] Improve Encoding::Registry.find tests
[v6.d REVIEW] Improve Encoding::Registry.find tests

Actually test it found the right encoding instead of giving us any
ol' Encoding object.

Orig: https://github.com/perl6/roast/commit/c8f9fa490

dogbert17/roast,dogbert17/roast,perl6/roast