net: Set Dummy net_if as default one for tests

If we run network tests under tests/net, then set the
Dummy network interface as a default one so that it will
be picked up first.

This should solve the issue if the DUT is having a real
network interfaces like onboard Ethernet interface which
could cause the test to fail. The test might fail in this
case because the network tests assume that only simulated
network interfaces are used by the tests.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>