testing: improve vnet support in pytest

• Allow vnet object to be directly referenced (self.vnet1 vs self.vnet_map["vnet1"])
• Allow iface object to be directly reference (vnet.bridge vs vnet.iface_alias_map["bridge"])
• Allow arbitrary interface alias names insted of ifX
• Add wait_objects_any() method for waiting object from multiple vnets
• Add wait() method for indefinite sleep on vnet handlers

MFC after: 2 weeks