fix(modem): Add CMUX example to target tests

components/esp_modem/examples/simple_cmux_client/pytest_cmux.py

@@ -0,0 +1,17 @@
+# SPDX-FileCopyrightText: 2023 Espressif Systems (Shanghai) CO LTD
+# SPDX-License-Identifier: Unlicense OR CC0-1.0
+from __future__ import print_function, unicode_literals
+
+
+def test_cmux_connection(dut):
+    """
+    steps:
+      1. initializes connection with SIM800
+      2. checks we get an IP
+      3. checks for the MQTT events
+    """
+    # 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')