ERP5Workflow: set variable expression correctly

in testERP5Workflow, a test was failing due to changes in
storage of expressions (we now store the expression object
so we don't have to recompile the expression all the time)

product/ERP5Workflow/tests/testERP5Workflow.py

@@ -126,7 +126,7 @@ class TestERP5Workflow(ERP5TypeTestCase):
     variable1 = workflow.newContent(portal_type='Workflow Variable',
                              title='actor')
     variable1.setReference('actor')
-    variable1.variable_expression = 'user/getUserName'
+    variable1.setVariableExpression('user/getUserName')
     self.getPortalObject().portal_types._getOb('Folder')\
       .edit(type_workflow_list=('wf'))
     doc = self.portal.newContent(portal_type='Folder', id='test_doc')