Fix ListPath for ADJ_IN and EnableFiltered=true and add tests.

Currently, even though `ApplyPolicy` is called for determining the
accepted routes after apply policy, the new route with attribute
modifications is not returned. This is problematic for gRPC API users.

Tests are added for all four cases that were described in
#2765. This PR makes the behaviour
correct for "Case/Attempt 2" described in the issue.

pkg/server/server.go

@@ -2627,8 +2627,10 @@ func (s *BgpServer) getAdjRib(addr string, family bgp.RouteFamily, in bool, enab
 					options := &table.PolicyOptions{
 						Validate: s.roaTable.Validate,
 					}
-					if s.policy.ApplyPolicy(peer.TableID(), table.POLICY_DIRECTION_IMPORT, path, options) == nil {
+					if p := s.policy.ApplyPolicy(peer.TableID(), table.POLICY_DIRECTION_IMPORT, path, options); p == nil {
 						filtered[path.GetNlri().String()] = path
+					} else {
+						adjRib.Update([]*table.Path{p})
 					}
 				}
 			}