Extend F1 Handover test

• do 1 more handover in the same test to verify robustness
  • This MR keeps a simple ping after the second handover since there are still issues due to scheduling starting too early, that is as soon as the UE context is added. This leads to retransmissions that could eventually lead to a test failure. The issue will be fixed in a future MR.
• add check on connected DU to verify successful handover
  • a new telnet function gets in input the UE ID and returns the ID of the connected DU
• wait some time after deploying DU to mimic time for measurements