Fixes#12560
When gracefully disconnecting from the coordinator, we would send the Disconnect message and then close the dRPC stream. However, closing the dRPC stream can cause the server not to process the Disconnect message, since we use the stream context in a `select` while sending it to the coordinator.
This is a product bug uncovered by the flake, and probably results in us failing graceful disconnect some minority of the time.
Instead, the `remoteCoordination` (and `inMemoryCoordination` for consistency) should send the Disconnect message and then wait for the coordinator to hang up (on some graceful disconnect timer, in the form of a context).
- `DERPForceWebSockets`: Test that DERP over WebSocket (as well as DERPForceWebSockets works). This does not test the actual DERP failure detection code and automatic fallback.
- `DERPFallbackWebSockets`: Test that falling back to DERP over WebSocket works.
Also:
- Rearranges some test code and refactors `TestTopology.StartServer` to be `TestTopology.ServerOptions` and take a struct instead of a function
Closes#13045
Spike Curtis