twister: check if output is connected to tty before 'stty sane' call

As of today we unconditionally call 'stty sane' that leads to
lots of error messages
'stty: standard input: Inappropriate ioctl for device'
when output isn't connected to tty (for example when we launch
twister in Jenkins)

Fix that by check if output is really connected to tty before
'stty sane' call.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>