CONTRIBUTING.adoc: add more content to the "Testing" section

CONTRIBUTING.adoc: add more content to the "Testing" section

This patch organizes `CONTRIBUTING.adoc`'s "Testing" section so that it
explains the basics of the testing environment first, and then
specificities of the Python bindings tests and guides.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I6f9afb0f847990eacd4a7cdf2bdf7401d6e02877
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1705
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>