tests: make lttng_live_server.py write temp port filename in same directory

tests: make lttng_live_server.py write temp port filename in same directory

When running lttng_live_server.py with `--port-filename yoyoyo`, I get:

    Traceback (most recent call last):
      File "lttng_live_server.py", line 1982, in <module>
        LttngLiveServer(port, port_filename, sessions, max_query_data_response_size)
      File "lttng_live_server.py", line 1716, in __init__
        self._write_port_to_file(port_filename)
      File "lttng_live_server.py", line 1839, in _write_port_to_file
        os.replace(tmp_port_file.name, port_filename)
    OSError: [Errno 18] Invalid cross-device link: '/tmp/tmpe70085jz' -> 'yoyoyo'

Fix this by placing the temporary file in the same directory as its
final location.

Change-Id: Ibe6fee5dc99860ab3cd4464a2f490e05c65df1eb
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10901
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>