tests: change lttng_live_server.py's --sessions-filename to be a positional argument

tests: change lttng_live_server.py's --sessions-filename to be a positional argument

When running lttng_live_server.py with no arguments, we get:

    Traceback (most recent call last):
      File "lttng_live_server.py", line 1968, in <module>
        trace_path_prefix,
      File "lttng_live_server.py", line 1871, in _session_descriptors_from_path
        with open(sessions_filename, "r") as sessions_file:
    TypeError: invalid file: None

The --sessions-filename argument is effectively mandatory.  Change it to
be a positional argument.

Change-Id: I48e67cbf1da8adfaa87d5024d91a922f9b67a7fd
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10898
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>