samples: lwm2m_client: re-add sanity check for WNC-M14A2A build

Instead of using the now removed overlay files, use the SHIELD=
command-line option for testing the WNC-M14A2A modem.

Signed-off-by: Michael Scott <mike@foundries.io>

samples/net/lwm2m_client/sample.yaml

@@ -18,3 +18,8 @@ tests:
     extra_args: OVERLAY_CONFIG=overlay-bt.conf
     platform_whitelist: nrf52_pca10040 nrf52840_pca10056 disco_l475_iot1
     tags: net lwm2m
+  sample.net.lwm2m_client.wnc_m14a2a:
+    harness: net
+    extra_args: SHIELD=wnc_m14a2a
+    platform_whitelist: frdm_k64f nrf52840_pca10056
+    tags: net lwm2m