Add Splunk log sink to the integration tests

We have a static GCP log sink configured with a test. We should also have a Splunk one.

What instance we reference needs design. We could run a static Splunk instance just for this, but preferably the static configuration in CB would reference a container instance spun up/set up just for the test.

Review the existing static GCP log sink configuration and its integration test first. Resolve how the Splunk instance should be referenced—static or container-based—then add the Splunk test and configuration so the integration test can run against that instance.

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