Make RegressionMixin the base to all builtins docs

Author: jjotero

docs/regression_test_api.rst

@@ -44,7 +44,7 @@ In essence, these builtins exert control over the test creation, and they allow
 Built-in types
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-.. py:function:: RegressionTest.parameter(values=None, inherit_params=False, filter_params=None)
+.. py:function:: RegressionMixin.parameter(values=None, inherit_params=False, filter_params=None)
 
   Inserts or modifies a regression test parameter.
   If a parameter with a matching name is already present in the parameter space of a parent class, the existing parameter values will be combined with those provided by this method following the inheritance behavior set by the arguments ``inherit_params`` and ``filter_params``.
@@ -89,7 +89,7 @@ Built-in types
      This only has an effect if used with ``inherit_params=True``.
 
 
-.. py:function:: RegressionTest.variable(*types, value=None)
+.. py:function:: RegressionMixin.variable(*types, value=None)
 
   Inserts a new regression test variable.
   Declaring a test variable through the :func:`variable` built-in allows for a more robust test implementation than if the variables were just defined as regular test attributes (e.g. ``self.a = 10``).