split tests of purchase_rfq_bid_workflow

purchase_requisition_bid_selection adds a field in the purchase order,
and sets a workflow transition to check it. That works fine in
production, i.e. both the field and the transition are available to
modules upstream (like purchase_requisition's generate_po() ).

On the other hand, when the tests of purchase_rfq_bid_workflow are run,
they do not see the fields added by modules downstream, but they see the
changes to the workflow, giving a test failure.