tests: net: coap: Enable loopback interface

There needs to be at least one network interface in order
to avoid this error.

E: There is no network interface to work with!

So enable loopback support which creates a loopback interface.

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

Author: jukkar

tests/net/lib/coap_server/common/prj.conf

@@ -4,6 +4,7 @@ CONFIG_NETWORKING=y
 CONFIG_NET_TEST=y
 CONFIG_ENTROPY_GENERATOR=y
 CONFIG_TEST_RANDOM_GENERATOR=y
+CONFIG_NET_LOOPBACK=y
 
 CONFIG_COAP=y
 CONFIG_COAP_SERVER=y