Add unit test for pfcwd interval (#259)

Why I did it
GCU has verified pfcwd interval config, we need to verify that GNMI can support the same pfcwd interval config.
Microsoft ADO: 27231852

How I did it
This unit test uses pfcwd interval config from GCU test.
This unit test generates GNMI request for pfcwd interval config 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

@@ -2619,6 +2619,31 @@ def create_checkpoint(file_name, text):
     }
 ]
 
+test_data_pfcwd_interval_patch = [
+    {
+        "test_name": "test_pfcwd_interval_config_updates",
+        "operations": [
+            {
+                "op": "update",
+                "path": "/sonic-db:CONFIG_DB/localhost/PFC_WD/GLOBAL/POLL_INTERVAL",
+                "value": "800"
+            }
+        ],
+        "origin_json": {
+            "PFC_WD": {
+                "GLOBAL": {}
+            }
+        },
+        "target_json": {
+            "PFC_WD": {
+                "GLOBAL": {
+                    "POLL_INTERVAL": "800"
+                }
+            }
+        }
+    }
+]
+
 class TestGNMIConfigDbPatch:
 
     def common_test_handler(self, test_data):
@@ -2775,3 +2800,10 @@ def test_gnmi_monitor_config_patch(self, test_data):
         Generate GNMI request for monitor config and verify jsonpatch
         '''
         self.common_test_handler(test_data)
+
+    @pytest.mark.parametrize("test_data", test_data_pfcwd_interval_patch)
+    def test_gnmi_pfcwd_interval_patch(self, test_data):
+        '''
+        Generate GNMI request for pfcwd interval and verify jsonpatch
+        '''
+        self.common_test_handler(test_data)