add a testing section to collections dev guide (#67370)

Signed-off-by: Adam Miller <admiller@redhat.com>

docs/docsite/rst/dev_guide/developing_collections.rst

@@ -171,7 +171,22 @@ TBD.
 tests directory
 ----------------
 
-TBD. Expect tests for the collection itself to reside here.
+Ansible Collections are tested much like Ansible itself, by using the
+`ansible-test` utility which is released as part of Ansible, version 2.9.0 and
+newer. Because Ansible Collections are tested using the same tooling as Ansible
+itself, via `ansible-test`, all Ansible developer documentation for testing is
+applicable for authoring Collections Tests with one key concept to keep in mind.
+
+When reading the :ref:`developing_testing` documentation, there will be content
+that applies to running Ansible from source code via a git clone, which is
+typical of an Ansible developer. However, it's not always typical for an Ansible
+Collection author to be running Ansible from source but instead from a stable
+release, and to create Collections it is not necessary to run Ansible from
+source. Therefore, when references of dealing with `ansible-test` binary paths,
+command completion, or environment variables are presented throughout the
+:ref:`developing_testing` documentation; keep in mind that it is not needed for
+Ansible Collection Testing because the act of installing the stable release of
+Ansible containing `ansible-test` is expected to setup those things for you.
 
 
 .. _creating_collections: