Fix: test: use BABELTRACE_BIN instead of babeltrace

Observed issue
==============

The System tests jobs fails on multi-session test since the move to bt2.

Cause
=====

The tests uses `babeltrace` instead of `BABELTRACE_BIN`.

Solution
========

Use `BABELTRACE_BIN`.

Add a babelrace bail out.

While there fix easy shellcheck warning.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I441d736e85c447c5765bffd520ec2f267c86048f