feat: 'extract' is used to replace 'output' when passing former teststep's (as a testcase) export value to next teststep

tests/testcases/create_user.json

@@ -0,0 +1,34 @@
+[
+  {
+    "config": {
+      "id": "create_user",
+      "variables": {
+        "device_sn": "TESTCASE_CREATE_XXX",
+        "uid": 9001
+      },
+      "output": [
+        "session_token"
+      ],
+      "base_url": "http://127.0.0.1:5000",
+      "name": "create user and check result."
+    }
+  },
+  {
+    "test": {
+      "testcase": "testcases/setup.yml",
+      "extract": [
+        "session_token"
+      ],
+      "name": "setup and reset all (override) for $device_sn."
+    }
+  },
+  {
+    "test": {
+      "testcase": "testcases/deps/check_and_create.yml",
+      "variables": {
+        "token": "$session_token"
+      },
+      "name": "create user and check result."
+    }
+  }
+]