Add test_gnoi_system.py with a simple test for System.Time. (#16334)

Description of PR
Create a new file for testing GNOI System API and bootstrap it with a simple test for System.Time

Approach
What is the motivation for this PR?
Create a new file for testing GNOI System API and bootstrap it with a simple test for System.Time

How did you do it?
Create a new file for testing GNOI System API and bootstrap it with a simple test for System.Time

How did you verify/test it?
Run on KVM

Author: hdwhdw

tests/gnmi/test_gnoi_system.py

@@ -0,0 +1,54 @@
+import pytest
+import logging
+import json
+
+from .helper import gnoi_request
+from tests.common.helpers.assertions import pytest_assert
+import re
+
+pytestmark = [
+    pytest.mark.topology('any')
+]
+
+
+"""
+This module contains tests for the gNOI System API.
+"""
+
+
+def test_gnoi_system_time(duthosts, rand_one_dut_hostname, localhost):
+    """
+    Verify the gNOI System Time API returns the current system time in valid JSON format.
+    """
+    duthost = duthosts[rand_one_dut_hostname]
+
+    # Get current time
+    ret, msg = gnoi_request(duthost, localhost, "Time", "")
+    pytest_assert(ret == 0, "System.Time API reported failure (rc = {}) with message: {}".format(ret, msg))
+    logging.info("System.Time API returned msg: {}".format(msg))
+    # Message should contain a json substring like this {"time":1735921221909617549}
+    # Extract JSON part from the message
+    msg_json = extract_first_json_substring(msg)
+    if not msg_json:
+        pytest.fail("Failed to extract JSON from System.Time API response")
+    logging.info("Extracted JSON: {}".format(msg_json))
+    pytest_assert("time" in msg_json, "System.Time API did not return time")
+
+
+def extract_first_json_substring(s):
+    """
+    Extract the first JSON substring from a given string.
+
+    :param s: The input string containing JSON substring.
+    :return: The first JSON substring if found, otherwise None.
+    """
+
+    json_pattern = re.compile(r'\{.*?\}')
+    match = json_pattern.search(s)
+    if match:
+        try:
+            return json.loads(match.group())
+        except json.JSONDecodeError:
+            logging.error("Failed to parse JSON: {}".format(match.group()))
+            return None
+    return None