Add test plugin which allows to query whether a module or action plugin is available (#3628)

* Add test which allows to query whether an action is available.

* Add documentation.

* Fix FreeBSD / macOS compatibility.

* Rename an_action -> a_module.

* Forgot one place.

docs/docsite/extra-docs.yml

@@ -3,3 +3,4 @@ sections:
   - title: Guides
     toctree:
       - filter_guide
+      - test_guide

docs/docsite/rst/test_guide.rst

@@ -0,0 +1,28 @@
+.. _ansible_collections.community.general.docsite.test_guide:
+
+community.general Test (Plugin) Guide
+=====================================
+
+The :ref:`community.general collection <plugins_in_community.general>` offers currently one test plugin.
+
+.. contents:: Topics
+
+Feature Tests
+-------------
+
+The ``a_module`` test allows to check whether a given string refers to an existing module or action plugin. This can be useful in roles, which can use this to ensure that required modules are present ahead of time.
+
+.. code-block:: yaml+jinja
+
+    - name: Make sure that community.aws.route53 is available
+      assert:
+        that:
+          - >
+            'community.aws.route53' is community.general.a_module
+
+    - name: Make sure that community.general.does_not_exist is not a module or action plugin
+      assert:
+        that:
+          - "'community.general.does_not_exist' is not community.general.a_module"
+
+.. versionadded:: 4.0.0