Two fixes to action plugins

* Fix the task_vars parameter to not default to a mutable type (dict)
* Implement invocation in the base class's run() method have each action
  module call the run() method's implemention in the base class.
* Return values from the action plugins' run() method takes the return
  value from the base class run() method into account so that invocation
  makes its way to the output.

Fixes #12869

test/integration/roles/test_good_parsing/tasks/main.yml

@@ -155,11 +155,9 @@
     that:
       - complex_param == "this is a param in a complex arg with double quotes"
 
-#- name: test variable module name
-#  action: "{{ variable_module_name }} msg='this should be debugged'"
-#  register: result
-#
-#- debug: var=result
+- name: test variable module name
+  action: "{{ variable_module_name }} msg='this should be debugged'"
+  register: result
 
 - name: assert the task with variable module name ran
   assert:

test/units/plugins/action/test_action.py

@@ -29,9 +29,15 @@ from ansible.plugins.action import ActionBase
 
 class TestActionBase(unittest.TestCase):
 
+    class DerivedActionBase(ActionBase):
+        def run(self, tmp=None, task_vars=None):
+            # We're not testing the plugin run() method, just the helper
+            # methods ActionBase defines
+            return dict()
+
     def test_sudo_only_if_user_differs(self):
         play_context = PlayContext()
-        action_base = ActionBase(None, None, play_context, None, None, None)
+        action_base = self.DerivedActionBase(None, None, play_context, None, None, None)
         action_base._connection = Mock(exec_command=Mock(return_value=(0, '', '')))
 
         play_context.become = True