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support/testing: let pexpect write stdout to log

When the parameter logfile is passed to spawn(), pexpect sends both
stdin and stdout to the logfile and it creates a double echo effect.

One way to avoid the double echo in the logfile would be to disable the
echo on the terminal just after login ("stty -echo"), but double echo of
user and password would remain.

Instead of that, send only the stdout to the logfile using the
logfile_read property.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>