Transforming to lowercase doesn't work for certain characters

[soletan]

The transformation to lowercase characters work locale-aware. However, at least for the German sz ligature there are known issues. A test revealed that this isn't related to hitchy-odem, but due to external causes. An issue has been opened with the party currently considered responsible with the issue.

Thus this ticket is basically just a local reminder to keep track of the linked issue.

[srl295]

Please track in nodejs/node#25738 as full-icu is just a data loader.