tests: name temporary files in test_live

I find it nice for debugging to have file names that have a little bit
of meaning.

Change-Id: I2505d5b363c2ea5f0c62255585e8898592d5ead0
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2111
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>