Add e2e test for the MQTT source

**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:](https://github.com/knative/eventing/blob/main/docs/personas.md)**
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.

Start with the MQTT source introduced in #7919 and the existing KinD e2e job setup. Add an independent job that installs an MQTT broker, creates a topic, connects the source through a container source to a sink, and verifies the MQTT source works end to end.

Written by the indexing model from the issue text.