Allow sources as unit test inputs and sinks as outputs

[lukesteensen]

Currently our unit test system deals only with transforms. This makes sense, as that's all we're able to actually test, but it does lead to some non-optimal configs in some cases. If a user has a source that fans out to multiple transform, it's likely that they want to provide test input as if it is coming from that source. This is currently not possible, so some users stick a dummy transform behind the source as a placeholder and target for those unit tests. This is an annoying thing for then to have to do, and hurts the performance of their config in the real world. A similar situation exists for sinks and unit test outputs.

While we can still only test transforms, we should add the ability for users to specify a source as the target for test input and a sink for test output. Internally, they can be simple aliases for the connected transform inputs/outputs.