net: ip: Increase the number of IPv4/IPv6 interfaces with loopback

In case loopback interface is enabled, it's most likely there will be
more than a one network interface if running on real hardware. Therefore
increase the default IPv4/IPv6 interface count to avoid the need to
increase it manually in every test suite.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>