[nrf fromtree] samples: net: sockets: echo_server: Add 802154-subg test

- Using beagleconnect freedom for testing subg overlay

Signed-off-by: Ayush Singh <[email protected]>
(cherry picked from commit 40d8926)

Author: Ayush1325

samples/net/sockets/echo_server/sample.yaml

@@ -142,3 +142,6 @@ tests:
       - native_sim/native/64
     extra_args:
       - OVERLAY_CONFIG="overlay-nsos.conf"
+  sample.net.sockets.echo_server.802154.subg:
+    extra_args: EXTRA_CONF_FILE="overlay-802154-subg.conf"
+    platform_allow: beagleconnect_freedom