Return list of instance as result of test tax wizard

Since 0db956eb2505 the on_change_with for xxx2Many must return a list of
instances.

Closes #12852
(grafted from 78704afb76ef28e16c518b99d2aee4c84eb01bc9)

--HG--
branch : 7.0

modules/account/tax.py

@@ -1928,8 +1928,7 @@ class TestTaxView(ModelView, TaxableMixin):
             for taxline in self._get_taxes():
                 del taxline['manual']
                 result.append(Result(**taxline))
-        self.result = result
-        return self._changed_values.get('result', [])
+        return result
 
 
 class TestTaxViewResult(ModelView):