tests/utils/utils.sh: add run_python helper function

tests/utils/utils.sh: add run_python helper function

Add a run_python function, which runs the passed command with
`tests/utils/python` in PYTHONPATH.  Re-implement run_python_bt2 on top
of run_python.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I30dcee2e85d581340881bc108e4cc8bbc65ba798
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10914
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>