support/testing/tests/package/test_python_paho_mqtt: new runtime test

This tests valdates that we can publish a message and read it back.

Signed-off-by: Marcus Hoffmann <marcus.hoffmann@othermo.de>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[yann.morin.1998@free.fr:
  - don't manually start mosquitto, there's a startup script for that
  - don't pass custom timeout
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

support/testing/tests/package/sample_python_paho_mqtt.py

@@ -0,0 +1,12 @@
+from paho.mqtt import publish, subscribe
+
+publish.single(
+        'buildroot/test',
+        payload="Hello, World!",
+        qos=2,
+        retain=True,
+        hostname="localhost",
+        port=1883)
+
+message = subscribe.simple('buildroot/test')
+print(message.payload.decode())