tests: delete temporary files in cli/test_help

tests: delete temporary files in cli/test_help

Also, use `-t` option of mktemp so that temporary files are created in
the system's tmp directory (/tmp) instead of the current working
directory.

Change-Id: If27b7cc420a70033abc81f8f901521ea58a830bc
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2106
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>