test_package.py: use list of public names instead of dedicated methods

This patch changes `test_package.py` so that the public names to verify
are listed in the `_public_names` list. The code creates the
corresponding test methods.

This makes it less cumbersome to add public name tests in the future.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I0d370d4b5fd4564000e1e0f273d3b1b36a8acc43
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2125
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>