tests: add `--port` option to lttng_live_server.py

tests: add `--port` option to lttng_live_server.py

This is useful when testing manually, to force using a specific port.
When not specified, we still use an OS-assigned port, so it doesn't
change the behavior when used by the testsuite.

Change-Id: Ic530bf13ddfa37fa1095c1e52a6c78f284e2c312
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8215
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10836
Tested-by: jenkins <jenkins@lttng.org>
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>