Re-use employee instance instead of reading from user

Get the employee from the user after having set it as current user return None
because the context of the instance still has the context of the admin user
(with employee=None) but applies to the user.
This is because set_user change globally the user and context of the config.

issue7144
review44831002

tests/scenario_purchase_requisition.rst

@@ -114,7 +114,7 @@ Create purchase requisition without product and description::
     >>> PurchaseRequisition = Model.get('purchase.requisition')
     >>> requisition = PurchaseRequisition()
     >>> requisition.description = 'Description'
-    >>> requisition.employee = requisition_user.employee
+    >>> requisition.employee = employee
     >>> requisition.supply_date = today
     >>> requisition_line = requisition.lines.new()
     >>> requisition_line.product = None
@@ -263,7 +263,7 @@ Try to delete requisition done::
 Delete draft requisition::
 
     >>> requisition = PurchaseRequisition()
-    >>> requisition.employee = requisition_user.employee
+    >>> requisition.employee = employee
     >>> requisition.supply_date = today
     >>> requisition.save()
     >>> PurchaseRequisition.delete([requisition])
@@ -273,7 +273,7 @@ Create purchase requisition with two different suppliers::
     >>> set_user(requisition_user)
     >>> requisition = PurchaseRequisition()
     >>> requisition.description = 'Description'
-    >>> requisition.employee = requisition_user.employee
+    >>> requisition.employee = employee
     >>> requisition.supply_date = today
     >>> requisition_line = requisition.lines.new()
     >>> requisition_line.description = 'Description'
@@ -320,7 +320,7 @@ Create purchase requisition then cancel::
     >>> set_user(requisition_user)
     >>> requisition = PurchaseRequisition()
     >>> requisition.description = 'Description'
-    >>> requisition.employee = requisition_user.employee
+    >>> requisition.employee = employee
     >>> requisition.supply_date = today
     >>> requisition_line = requisition.lines.new()
     >>> requisition_line.description = 'Description'
@@ -334,7 +334,7 @@ Create purchase requisition, wait then reject::
     >>> set_user(requisition_user)
     >>> requisition = PurchaseRequisition()
     >>> requisition.description = 'Description'
-    >>> requisition.employee = requisition_user.employee
+    >>> requisition.employee = employee
     >>> requisition.supply_date = today
     >>> requisition_line = requisition.lines.new()
     >>> requisition_line.description = 'Description'