test: Rename interfaces defined in virtual networks test

This work is similar to one performed for ./net/ptp/clock/ test.

However, there is one notable extension - the check in iface_cb()
callback also checks for virtual interfaces defined in
subsys/net/l2/virtual/ipip/ipip.c. Those 'virtual' interfaces are
required for this test to be run.

Signed-off-by: Lukasz Majewski <lukma@denx.de>