Add unit test for pg headroom (#261)

Why I did it
GCU has verified pg headroom config, we need to verify that GNMI can support the same pg headroom config.
Microsoft ADO: 27231872

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

@@ -2717,6 +2717,33 @@ def create_checkpoint(file_name, text):
     }
 ]
 
+test_data_pg_headroom_patch = [
+    {
+        "test_name": "test_pg_headroom_update",
+        "operations": [
+            {
+                "op": "replace",
+                "path": "/sonic-db:CONFIG_DB/localhost/BUFFER_PROFILE/pg_lossless_100000_300m_profile/xoff",
+                "value": "160001"
+            }
+        ],
+        "origin_json": {
+            "BUFFER_PROFILE": {
+                "pg_lossless_100000_300m_profile": {
+                    "xoff": "160000"
+                },
+            }
+        },
+        "target_json": {
+            "BUFFER_PROFILE": {
+                "pg_lossless_100000_300m_profile": {
+                    "xoff": "160001"
+                },
+            }
+        }
+    }
+]
+
 class TestGNMIConfigDbPatch:
 
     def common_test_handler(self, test_data):
@@ -2887,3 +2914,10 @@ def test_gnmi_pfcwd_status_patch(self, test_data):
         Generate GNMI request for pfcwd status and verify jsonpatch
         '''
         self.common_test_handler(test_data)
+
+    @pytest.mark.parametrize("test_data", test_data_pg_headroom_patch)
+    def test_gnmi_pg_headroom_patch(self, test_data):
+        '''
+        Generate GNMI request for pg headroom and verify jsonpatch
+        '''
+        self.common_test_handler(test_data)