add output:

1, change return value of running testset;
2, print extracted variables at the end of testset.

tests/data/demo_testset_layer.yml

@@ -10,6 +10,8 @@
         headers:
             Content-Type: application/json
             device_sn: $device_sn
+    output:
+        - token
 
 - test:
     name: get token