Testing IS-07 MQTT

When testing IS-07 MQTT implementations, there are some configuration issues to consider.

The testing tool does not launch its own MQTT broker. The broker to be used by the node under test must be configured and launched separately.
Automatic discovery of the MQTT broker by nodes relies on an instance of the DNS-SD service type _nmos-mqtt._tcp being advertised, as specified by IS-07.
- When the DNS_SD_MODE is set to 'multicast', the MQTT broker should be advertised via mDNS. If the broker does not do so itself, the advertisements should be configured manually. For example, when running a broker on a Linux host with the Avahi utilities installed, a static service definition may be configured, or an equivalent avahi-publish command may be used, like:
```
avahi-publish -s mqttrelay _nmos-mqtt._tcp -d local 1883 api_proto=http api_auth=false &
```
  Or with mDNSResponder installed, on Linux or Windows, the dns-sd tool may be used, like so:
```
dns-sd -R mqttrelay _nmos-mqtt._tcp local 1883 api_proto=http api_auth=false &
```
- When the DNS_SD_MODE configuration parameter is set to 'unicast' to test with unicast discovery, ENABLE_MQTT_BROKER may be set to True to add a broker to the testing tool's own mock DNS server. Each of the parameters which begins MQTT_BROKER_ should be configured correctly.

The test suite must be able to connect to the broker(s) specified in each sender's active transport parameters and subscribe to the relevant topics.

The test suite requires the broker to support MQTT version 5, though the node under test may use version 3.1.1.
The config parameters MQTT_USERNAME and MQTT_PASSWORD may be changed to specify authentication details for connecting to the broker.

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