Starting with late Windows 10 version, in ucrtbase, stderr is now unbuffered whatever the type of the underlying fd (previously, native only set it to unbuffered when attached to character fd (console, NUL...)).
This serie adds also tests for msvcrt & ucrtbase to show the discrepancies.
Note: ucrtbase's tests also include a reversed engineered structure layout for FILE. _get_stream_buffer_pointers already gives base, ptr and cnt, and toying with setvbuf gave easily the rest of the fields. Didn't spend time in guessing the flags meaning: which look different from msvcrt's.
Related to https://bugs.winehq.org/show_bug.cgi?id=56459 @piotr: I can share the details (the access to the test result in the bug report is limited).
-- v4: ucrtbase: Let stderr be always be unbuffered. ucrtbase/tests: Add tests for checking buffering state of standard streams. msvcrt/tests: Add tests for check buffering state of standard streams.