test: set subscriber credentials explicitly to match mosquitto testuser/testpass

test/docker-compose.yml

@@ -50,8 +50,8 @@ services:
     environment:
       - MQTT_BROKER=mosquitto:1883
       - MQTT_TOPIC=lambdaiot
-      - MQTT_USERNAME=${MOSQ_USER}
+      - MQTT_USERNAME=testuser
-      - MQTT_PASSWORD=${MOSQ_PASS}
+      - MQTT_PASSWORD=testpass
     command: sh -c "pip install paho-mqtt && python subscribe.py"
     depends_on:
       - mosquitto