[TESTS] Test Bonding.ConnectNonBlocking needs refinement, or a code fix is needed

[ethouris]

By unknown reason, sometimes ConnectNonBlocking is not reporing UPDATE event on the second connection. Might be that the UPDATE state is outdated because the connections are known already, or both listeners are ready to extract the connection for the group. A failure with unset UPDATE event after accepting a group has been once found on Travis.