fix(modem): Examples: use local configs for MQTT topic/data

To avoid issues in CI, as we're using public server
and could receive data by people playing with the example

components/esp_modem/examples/simple_cmux_client/pytest_cmux.py

@@ -1,4 +1,4 @@
-# SPDX-FileCopyrightText: 2023 Espressif Systems (Shanghai) CO LTD
+# SPDX-FileCopyrightText: 2023-2024 Espressif Systems (Shanghai) CO LTD
 # SPDX-License-Identifier: Unlicense OR CC0-1.0
 from __future__ import print_function, unicode_literals
 
@@ -10,8 +10,17 @@ def test_cmux_connection(dut):
       2. checks we get an IP
       3. checks for the MQTT events
     """
+    # Get topic and data from Kconfig
+    topic = ''
+    data = ''
+    try:
+        topic = dut.app.sdkconfig.get('EXAMPLE_MQTT_TEST_TOPIC')
+        data = dut.app.sdkconfig.get('EXAMPLE_MQTT_TEST_DATA')
+    except Exception:
+        print('ENV_TEST_FAILURE: Cannot find broker url in sdkconfig')
+        raise
     # Check the sequence of connecting, publishing, disconnecting
     dut.expect('Modem has correctly entered multiplexed')
     # Check for MQTT connection and the data event
-    dut.expect('TOPIC: /topic/esp-modem')
-    dut.expect('DATA: Hello modem')
+    dut.expect(f'TOPIC: {topic}')
+    dut.expect(f'DATA: {data}')