This approach has several benefits:
* We end up with ip/tun setup output in the same log file as
open5gs-upfd process output.
* We configure all ip/tun *before* the open5gs-upfd process starts.
Furthermore, we have same procedure as in ttcn3-ggsn-tests-ogs, which
simplifies maintainment/use.
The IP address pool for UEs is still different in pgw-tests and
ggsn-tests-ogs. We can make them the same in subsequent patches.
Change-Id: I94219abbeb5e004ce707407b5aa5ee8ad6c3a80e
