Linux setlocale() can report C when ICU is using en as locale

Setting LC_COLLATE=en.UTF-8 LANG=en.UTF-8 LC_ALL=en.UTF-8 LANGUAGE=en in my environment and running tst_QString::localeAwareCompare() reveals that setlocale(LC_COLLATE, nullptr) and setlocale(LC_ALL, nullptr) both return "C"; but ICU is (correctly) using the collation of English. In contrast, when I s/en/C/ in all those environment variables, ICU (again correctly) uses the C locale.
This makes it tricky for the relevant test to determine whether it's really using the C locale, since the backend it's delegating to is ICU.