MBS-14050: Add tests for find_best_primary_alias

[reosarevok]

Implement MBS-14050

Description

I think I covered all the main cases here: exact locale match, fallback to language default, fallback to English, skipping non-primary aliases, skipping ended aliases. Happy to add more cases if I missed something :)

[derat]

Nice!

It might be worthwhile passing different alias lists to test what happens when there are no primary aliases at all, e.g.

• Alias exists for requested locale, but no aliases are primary
• Alias exists for different locale with same language, but no aliases are primary
• Alias exists for English, but no aliases are primary

(Maybe those are already covered by the existing "no aliases" case if the code starts by filtering out all non-primary aliases, though.)

Might also be worth testing what happens when there's no English primary alias to fall back to, too.