`ChannelTurbine` uses unstable APIs via `@OptIn`

[JakeWharton]

We need to pursue stabilization upstream and track it.

• SendChannel.isClosedForSend
  • Consider stabilizing SendChannel.isClosedForSend Kotlin/kotlinx.coroutines#3448
  • Stable + delicate in coroutines 1.7.0 ✅
• TestCouroutineScheduler
  • Stable in coroutines 1.7.0 ✅
• UnconfinedTestDispatcher
  • Tracking issue for stabilization of UnconfinedTestDispatcher Kotlin/kotlinx.coroutines#3749