twister: Set stdin explicitly on stty calls

stty operates on the file descriptor from stdin, but we want to operate
on stdout. If stdin and stdout are not the same tty then stty prints an
error.

Redirect stdout to stdin for stty calls.

Tested by running twister -T tests/subsys/shell </dev/null

Signed-off-by: Jeremy Bettis <jbettis@google.com>