tests: conditionalize an error message about unlinking a non empty directory
The message on Windows comes from win32.unlink(). It looks like os.unlink() on
posix platforms is a simple call to unlink(3), which turns into unlinkat(2).
Since there's a comment in one of the tests that the message should be improved,
I don't think it's worth adding a check in win32.unlink() to see if it's empty,
if that function is always going to fail on a directory. (It seems like the
POSIX spec allows unlinking directories though.)