Fix: tests: use run_python in gen_mctf_trace

Fix: tests: use run_python in gen_mctf_trace

gen_mctf_trace calls the python3 interpreter, without using the
`run_python` wrapper.  PYTHONPATH is therefore not set appropriately
for Python to find our local `typing.py` module, if needed.

Add a call to `run_python` to fix that.

Change-Id: Id813169905d511e5600a70cebad681b00ebb49d8
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11171
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
CI-Build: Michael Jeanson <mjeanson@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>