- `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
