Add unit test for ECN config (#250)

Why I did it
GCU has verified ECN config, we need to verify that GNMI can support the same ECN config.
Microsoft ADO: 27231797

How I did it
This unit test uses ECN config from GCU test.
This unit test generates GNMI request for DHCP relay and use jsonpatch to verify patch file generated by GNMI server.

How to verify it
Run GNMI unit test.

test/test_gnmi_configdb_patch.py

@@ -1704,6 +1704,47 @@ def create_checkpoint(file_name, text):
     }
 ]
 
+test_data_ecn_config_patch = [
+    {
+        "test_name": "test_ecn_config_updates",
+        "operations": [
+            {
+                "op": "replace",
+                "path": "/sonic-db:CONFIG_DB/localhost/WRED_PROFILE/AZURE_LOSSLESS/green_min_threshold",
+                "value": "2000001"
+            },
+            {
+                "op": "replace",
+                "path": "/sonic-db:CONFIG_DB/localhost/WRED_PROFILE/AZURE_LOSSLESS/green_max_threshold",
+                "value": "10000001"
+            },
+            {
+                "op": "replace",
+                "path": "/sonic-db:CONFIG_DB/localhost/WRED_PROFILE/AZURE_LOSSLESS/green_drop_probability",
+                "value": "6"
+            },
+        ],
+        "origin_json": {
+            "WRED_PROFILE": {
+                "AZURE_LOSSLESS": {
+                    "green_min_threshold": "2000000",
+                    "green_max_threshold": "10000000",
+                    "green_drop_probability": "5"
+                }
+            }
+        },
+        "target_json": {
+            "WRED_PROFILE": {
+                "AZURE_LOSSLESS": {
+                    "green_min_threshold": "2000001",
+                    "green_max_threshold": "10000001",
+                    "green_drop_probability": "6"
+                }
+            }
+        }
+    }
+]
+
 class TestGNMIConfigDbPatch:
 
     def common_test_handler(self, test_data):
@@ -1804,3 +1845,10 @@ def test_gnmi_dynamic_acl_patch(self, test_data):
         Generate GNMI request for dynamic acl and verify jsonpatch
         '''
         self.common_test_handler(test_data)
+
+    @pytest.mark.parametrize("test_data", test_data_ecn_config_patch)
+    def test_gnmi_ecn_config_patch(self, test_data):
+        '''
+        Generate GNMI request for ecn config and verify jsonpatch
+        '''
+        self.common_test_handler(test_data)