test_convert_args: test multiple leftover arguments

test_convert_args: test multiple leftover arguments

When `babeltrace convert` is given multiple leftover arguments:

    babeltrace /path/to/trace1 /path/to/trace2 /path/to/trace3

it configures a single `src.ctf.fs` component with a `paths` array
parameter containing the three paths.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I0c19c730bc276dba7160b9efcb72dc8077c078f5