Tracking issue for stabilization of UnconfinedTestDispatcher

[JakeWharton]

UnconfinedTestDispatcher didn't get the stable treatment in #3622. This is a tracking issue for its stabilization at some point in the future.

For us, this is the final remaining unstable API preventing Turbine from going 1.0. Turbine collects flows in a coroutine which was traditionally launched on the UNCONFINED dispatcher. This is to ensure we do not miss any emitted values from a flow when it performs some operation on backpressure (conflating, dropping, etc.). With the new runTest infrastructure, we were forced to take on a 'kotlinx-coroutines-test' dependency in order to conditionally use UnconfinedTestDispatcher wrapped around the caller's TestCoroutineScheduler so as to not break virtual time.