tests: Move closed_std* tests out of the POSIX group

tests: Move closed_std* tests out of the POSIX group

POSIX actually says

> If the utility would be executed with file descriptor 0, 1, or 2
> closed, implementations may execute the utility with the file
> descriptor open to an unspecified file.

So we're not guaranteed to be able to detect the situation in the first
place.  Add a best-effort check for these platforms and skip the test.

Link: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_01_01