cluts: Incorrect test expectations in buf module (errno)

[Rich Felker <dalias@aerifal.cx>]

Hi,
It seems cluts' buf module is surprised that strerror_r and ttyname_r
did not set errno to ERANGE. However these functions are not specified
to report anything in errno. Instead they return the error code
directly. This is an unfortunate inconsistency created in the early
days of "reentrant" (_r) functions due to confusion over whether errno
could be safely used in multithreaded programs. Note that some other
functions which return error codes directly are:

posix_* (posix_memalign etc.)
pthread_*

Rich