Enhance diff_check with more unicode et al

Follow up action item from a topic I raised in https://github.com/rust-lang/rustfmt/pull/5864#issuecomment-1676425492 relative to adding some additional test repos to improve our coverage of string formatting with various characters and encodings.

One suggested repo to include is https://github.com/unicode-org/icu4x as well as some from https://github.com/unicode-rs

I opted to simply mention icu4x as part of this instead of directly adding it because I noticed that repo is using cargo make and I didn't have bandwidth to determine if that'd cause any issues/require any additional changes to our script.

New repos can be wired up by adding a call site with respective args here:

https://github.com/rust-lang/rustfmt/blob/b069aac44ddfdb70d55d9ae40695be44515e5bb0/ci/check_diff.sh#L170-L192