samples: echo_server: Test overlay-smsc911x.conf via sanitycheck

By adding relevant test config to sample.yaml.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>

Author: pfalcon

samples/net/echo_server/sample.yaml

@@ -50,3 +50,7 @@ tests:
     extra_args: OVERLAY_CONFIG="overlay-enc28j60.conf"
     tags: net enc28j60
     platform_whitelist: arduino_101
+  test_smsc911x:
+    extra_args: OVERLAY_CONFIG="overlay-smsc911x.conf"
+    tags: net
+    platform_whitelist: mps2_an385