Docs how to test (2nd) (#24094)

* Big testing doc refactor
* Combine all the testing documentation in to one place to make it easier to find
* Convert everything to RST
* Create testing_network guide
* Create testing landing page
* For each section detail "how to run" and "how to extend testing"
* More examples
* Lots more detail

docs/docsite/rst/dev_guide/testing_units.rst

@@ -0,0 +1,93 @@
+**********
+Unit Tests
+**********
+
+Unit tests are small isolated tests that target a specific library or module.
+
+.. contents:: Topics
+
+Available Tests
+===============
+
+Unit tests can be found in `test/units <https://github.com/ansible/ansible/tree/devel/test/units>`_, notice that the directory structure matches that of ``lib/ansible/``
+
+Running Tests
+=============
+
+Unit tests can be run across the whole code base by doing:
+
+.. code:: shell
+
+    cd /path/to/ansible/source
+    source hacking/env-setup
+    ansible-test units --tox
+
+Against a single file by doing:
+
+.. code:: shell
+
+   ansible-test units --tox apt
+
+Or against a specific Python version by doing:
+
+.. code:: shell
+
+   ansible-test units --tox --python 2.7 apt
+
+
+
+For advanced usage see the online help::
+
+   ansible-test units --help
+
+
+Installing dependencies
+=======================
+
+``ansible-test`` has a number of dependencies , for ``units`` tests we suggest using ``tox``
+
+The dependencies can be installed using the ``--requirements`` argument. For example:
+
+.. code:: shell
+
+   ansible-test units --tox --python 2.7 --requirements apt
+
+
+.. note:: tox version requirement
+
+   When using ``ansible-test`` with ``--tox`` requires tox >= 2.5.0
+
+
+The full list of requirements can be found at `test/runner/requirements <https://github.com/ansible/ansible/tree/devel/test/runner/requirements>`_. Requirements files are named after their respective commands. See also the `constraints <https://github.com/ansible/ansible/blob/devel/test/runner/requirements/constraints.txt>`_ applicable to all commands.
+
+
+Extending unit tests
+====================
+
+
+.. warning:: What a unit test isn't
+
+   If you start writing a test that requires external services then you may be writing an integration test, rather than a unit test.
+
+Fixtures files
+``````````````
+
+To mock out fetching results from devices, you can use ``fixtures`` to read in pre-generated data.
+
+Text files live in ``test/units/modules/network/PLATFORM/fixtures/``
+
+Data is loaded using the ``load_fixture`` method
+
+See  `eos_banner test <https://github.com/ansible/ansible/blob/devel/test/units/modules/network/eos/test_eos_banner.py>`_ for a practical example.
+
+Code Coverage
+`````````````
+
+Most ``ansible-test`` commands allow you to collect code coverage, this is particularly useful when to indicate where to extend testing.
+
+To collect coverage data add the ``--coverage`` argument to your ``ansible-test`` command line:
+
+.. code:: shell
+
+    ansible-test units --coverage
+    ansible-test coverage html