tests: make --port-filename optional in lttng_live_server.py

While port_filename is useful for automation, it isn't essential when
using lttng_live_server.py manually.  Make that argument non-required.

Also, print a message indicating the port used to listen, which is handy
for interactive use.

Change how lttng_live_server.py is invoked to redirect stdout to stderr,
hopefully avoiding messing up the tap output format.

Change-Id: Ie185ced440c51fb1bb8355553f7b4a7725a13542
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10896
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>