Add e2e test for the MQTT source

[Cali0707]

Problem
We recently introduced an MQTT source in #7919, but we do not currently have any tests for this. Ideally, it would be a separate job that can run in KinD so that it is straightforward to test and independent of other tests.

We need to:

• Install an MQTT broker
• Create a topic
• Point the source image with a container source to a sink

Persona:
Which persona is this feature for?

Exit Criteria
An e2e test verifying the MQTT source works

Time Estimate (optional):
How many developer-days do you think this may take to resolve? 2-3

Additional context (optional)
Add any other context about the feature request here.

[Cali0707]

cc @pierDipi @creydr not sure what you guys think about having a test that would require setting up MQTT, but also we have the MQTT container source now so I think it is worth testing...

[creydr]

+1 on adding an e2e test. But IMO we should then also think about moving this to a separate repo (as we have for the other "custom" sources - like eventing-github)