tests: sprintf: cleanup to work with newlib

newlib doesn't implement the internal buffer the same way that minimal
libc does, so only run that check with min libc (ie !CONFIG_NEWLIB_LIBC).
Also, reported the length we did get if the buffer is to big.  Finally
include <stdarg.h> since we are dealing with va_lists and such.

Change-Id: I6b23e448e5785df978ac8c2757099e2b8aaace54
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>