tests/lib/conds: make `condMain()` take a span for args

Make `argv` a span, instead of having `argc` + `argv`.  There is no
point in this change except that it seemed possible and I wanted to do
it.

Change-Id: I1e7e76638e9e6365df05bb5c7b175d44b6badf6f
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12118
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>