bt2: update bindings to make test_plugins pass

bt2: update bindings to make test_plugins pass

This patch updates the necessary stuff in the bindings to make
test_plugins pass.

One new test is added, test_find_file, which tests passing the path to a
plugin shared object to bt2.find_plugins.

Change-Id: Ie2ac6e830fb18d8839d2879567509cd2f6abbd81
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1087
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins