tests: don't swallow stderr when running babeltrace CLI

I have found it very difficult to investigate failing tests if we don't
show stderr, so I suggest we just let it get printed.  Some tests are
expected to print to stderr in their normal course of operation (because
they test babeltrace error cases), so it means there will be some
additional output in a normal test run.  But I think it's a fair
trade-off.

Change-Id: Ia9bc5a706dd756ce0c9d1be4fec7510b12dface7
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1609
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>