tests: add bt_cli util function

tests: add bt_cli util function

Add a function that simply runs the CLI, redirecting stdout and
stderr to the specified files.  This will allow more flexible tests than
the existing "diff" function.

Change-Id: I8319c4a74a0baa3b40afb871bfb0605ac81c3270
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1837
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>