Ensure that the UE has an IP address before continuing the test.
Otherwise, it might happen we ping before the UE is connected.
Do not do this in do-ra and phytest, as they use --noS1, which implies
statically assigned IP address, so IP address check always succeeds(!).
The 5G-RFsim F1 test tested multiple DUs at one CU. This
functionality is retained in the new F1+E1 test.
The 5G-RFsim E1 test tested a single CU-UP at a single CU-CP. Here, we
extend to use multiple CU-UPs, one for each DU. The associated is
ensured by using three different F1-U transport networks, one for each
DU/CU-UP pair.
Robert Schmidt