tests: net: socket: udp: Don't use NET_CONFIG_SETTINGS

As the UDP test suite is mostly intended to run over loopback, use
loopback addresses and skip configuration of other addresses.

For the test cases that use fake Ethernet iterface the configuration
is done manually anyway, so just rename the symbols to avoid
collission.

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