tests: cleanup cli/convert/test_convert_args

tests: cleanup cli/convert/test_convert_args

A few cleanups:

- Use bt_cli instead of running the babeltrace binary by hand.
- Give a base name to the temporary files.
- Split CLI arguments into an array before passing them to bt_cli (this
  avoids a shellcheck warning).

bt_cli prints the executed command line, so it becomes unnecessary to
print it manually in this test, which allows simplifying things a bit.

Change-Id: I043099b460ba4ab428beb305de4fe0ad51c20de7
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3142
Tested-by: jenkins <jenkins@lttng.org>