bpo-35330: Don't call the wrapped object if side_effect is set (GH10973)

* tests: Further validate `wraps` functionality in `unittest.mock.Mock`

Add more tests to validate how `wraps` interacts with other features of
mocks.

* Don't call the wrapped object if `side_effect` is set

When a object is wrapped using `Mock(wraps=...)`, if an user sets a
`side_effect` in one of their methods, return the value of `side_effect`
and don't call the original object.

* Refactor what to be called on `mock_call`

When a `Mock` is called, it should return looking up in the following
order: `side_effect`, `return_value`, `wraps`. If any of the first two
return `mock.DEFAULT`, lookup in the next option.

It makes no sense to check for `wraps` returning default, as it is
supposed to be the original implementation and there is nothing to
fallback to.

Lib/unittest/mock.py

@@ -1027,28 +1027,27 @@ class CallableMixin(Base):
                 break
             seen.add(_new_parent_id)
 
-        ret_val = DEFAULT
         effect = self.side_effect
         if effect is not None:
             if _is_exception(effect):
                 raise effect
-
-            if not _callable(effect):
+            elif not _callable(effect):
                 result = next(effect)
                 if _is_exception(result):
                     raise result
-                if result is DEFAULT:
-                    result = self.return_value
+            else:
+                result = effect(*args, **kwargs)
+
+            if result is not DEFAULT:
                 return result
 
-            ret_val = effect(*args, **kwargs)
+        if self._mock_return_value is not DEFAULT:
+            return self.return_value
 
-        if (self._mock_wraps is not None and
-             self._mock_return_value is DEFAULT):
+        if self._mock_wraps is not None:
             return self._mock_wraps(*args, **kwargs)
-        if ret_val is DEFAULT:
-            ret_val = self.return_value
-        return ret_val
+
+        return self.return_value