miner: use channels instead of atomics in update loop (#21536)

This PR changes several different things:

- Adds test cases for the miner loop
- Stops the worker if it wasn't already stopped in worker.Close()
- Uses channels instead of atomics in the miner.update() loop

Co-authored-by: Felix Lange <fjl@twurst.com>

miner/worker.go

@@ -303,6 +303,7 @@ func (w *worker) isRunning() bool {
 // close terminates all background threads maintained by the worker.
 // Note the worker does not support being closed multiple times.
 func (w *worker) close() {
+	atomic.StoreInt32(&w.running, 0)
 	close(w.exitCh)
 }