tests: fix warning when checking ospfv3 convergence

tests: fix warning when checking ospfv3 convergence

The following error is shown when running the OSPFv3 tests

2021-03-16 23:37:44,792 INFO: Function returned global name 'data_rid' is not defined
2021-03-16 23:37:44,792 INFO: Retry [#1] after sleeping for 2s
2021-03-16 23:37:46,794 INFO: Verifying OSPF6 neighborship on router r1:
2021-03-16 23:37:46,993 INFO: Output for command [ show ipv6 ospf6 neighbor ] on router r1:
Neighbor ID     Pri    DeadTime    State/IfState         Duration I/F[State]
2.2.2.2           1    00:00:03     Full/PointToPoint    00:00:01 r1-r2-eth0[PointToPoint]

Fix the "data_rid" warning by using the correct variable

Signed-off-by: ckishimo <carles.kishimoto@gmail.com>