CONTRIBUTING.adoc: add how to run `bt2` tests

This commit adds a tiny testing section containing an introduction and a
few useful commands to run the `bt2` tests.

Philippe will without a doubt rework this section upon the release of
Babeltrace 2.0. So this commit simply adds commands useful to developers
in the meantime.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: If4be315ddcc09efe9224e42de62c5b01c73ead8f
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1602
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>