Make RFsim start up faster by starting core and RAN start at the same
time. This speeds up because we do not wait for "healthy" containers
serially, but parallely.

For the RAN, it is safe to start gNB and UE at the same time in SA mode
(as the UE will only attach once the radio, and hence the full gNB, is
ready).

We do not start in parallel in do-ra and phytest, as in these modes the
gNB first has to write a UE-specific configuration file. If we start
both gNB and UE at the same time, the UE might try to read the file
before it is written, leading to a failure.