test/test-shutdown.py: optionally display the test I/Os in a dedicated log file

test/test-shutdown.py: optionally display the test I/Os in a dedicated log file

Given that the test involves screen(1), sending various control sequences to
resize/clear the screen, most of the logs sent from the python script were
nearly impossible to read or mixed with other messages sent to the console
hence making the debug harder when the test is run manually.

This patch introduces an option to redirect the pexpect IOs into a file (to be
used in $STATEDIR/TEST-69-SHUTDOWN/run-nspawn).

The pexpect logs are also enabled later so the boot logs are skipped since
those are already included in the journal.