Extend MQTT tests (#6648)

tests/components/mqtt/test.esp8266.yaml

@@ -1,15 +1,2 @@
-wifi:
-  ssid: MySSID
-  password: password1
-
-mqtt:
-  broker: test.mosquitto.org
-  port: 1883
-  discovery: true
-  discovery_prefix: homeassistant
-  log_topic:
-  on_message:
-    topic: testing/sensor/testing_sensor/state
-    qos: 0
-    then:
-      - logger.log: Mqtt Test
+packages:
+  common: !include common.yaml