K8s: integration tests for the k8s source

[MOZGIII]

Implement integration tests for the k8s source.

To address together with #2631.

Part of #2222.

Since every individual building block for the k8s implementation is to be thoroughly tested, only a simple smoke test is required in the integration test. Mainly to ensure the glue code works accordingly, and that we didn't go out-of-spec somewhere in our implementation.

Things to test:

• log files selection (aka filtering);
• format parsing, including automatic partial merges;
• pod metadata annotations;
• race conditions around pod deletion (feat(new source): Initial kubernetes_logs implementation #2653 (comment)).