Add coverage for LanguagesHelper#locale_name_for_sorting method

[mjankowski]

Rounding out various code paths.

[ClearlyClaire]

Honestly I'm not even sure what the intent was behind those things, so I'm not sure this makes sense codifying them in a test.

[mjankowski]

Fair enough- looks like they were added as part of a sorting change - https://github.com/mastodon/mastodon/pull/27158/files

My assumption was that there was some (valid) case where the inbound locale was either blank/nil/undefined or something and we wanted it to sort first.

If that's not true or this is handling an edge case that doesn't exist, I can remove the lines instead.

[ClearlyClaire]

This is only ever called with LanguagesHelper::SUPPORTED_LOCALES.keys or I18n.available_locales as keys, so the blank case and und are never going to occur I think? And a language code being unknown is technically possible if we are not consistent when adding new entries, but should otherwise not happen either