Add e2e test for ApiServerSource node selector

[Cali0707]

Problem
After #7584 is merged, we should add an e2e test to verify that it is working correctly.

Persona:
Which persona is this feature for?

Exit Criteria
An e2e test in the rekt folder which verifies that the node selector works. My initial thoughts would be a test that:

1. Adds a label to one of the nodes in the cluster
2. Created an ApiServerSource which has the nodeselector set to select for that label
3. Gets the pod(s) associated with the ApiServerSource deployment and verifies that they are all on the correct node.

Time Estimate (optional):
How many developer-days do you think this may take to resolve? 1-2

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

[sadath-12]

/assign