Add a (disabled) code-smell test for detecting _ variables

We are reserving the _ identifier for i18n work.  Code should use the
identifier dummy for dummy variables instead.

This test is currently skipped as someone needs to generate the list of
files which are currently out of compliance before this can be turned
on.

docs/docsite/rst/dev_guide/testing/sanity/no-underscore-variable.rst

@@ -0,0 +1,28 @@
+Sanity Tests » no-underscore-variable
+=====================================
+
+In the future, Ansible may use the identifier ``_`` to internationalize its
+message strings.  To be ready for that, we need to make sure that there are
+no conflicting identifiers defined in the code base.
+
+In common practice, ``_`` is frequently used as a dummy variable (a variable
+to receive a value from a function where the value is useless and never used).
+In Ansible, we're using the identifier ``dummy`` for this purpose instead.
+
+Example of unfixed code:
+
+.. code-block:: python
+
+    for _ in range(0, retries):
+        success = retry_thing()
+        if success:
+            break
+
+Example of fixed code:
+
+.. code-block:: python
+
+    for dummy in range(0, retries):
+        success = retry_thing()
+        if success:
+            break