Wait for workers to stop in TestPostVerifierScaling (#5525)

## Motivation

Unit test `TestPostVerifierScaling` is flaky because the test doesn't wait for the workers to stop after scaling the pool down.

activation/post_verifier.go

@@ -78,6 +78,7 @@ type postVerifierWorker struct {
 	prioritized <-chan *verifyPostJob
 	jobs        <-chan *verifyPostJob
 	stop        chan struct{} // signal to stop this worker
+	stopped     chan struct{} // signal that this worker has stopped
 	shutdown    chan struct{} // signal that the verifier is closing
 }
 
@@ -215,6 +216,7 @@ func (v *offloadingPostVerifier) scale(target int) {
 				prioritized: v.prioritized,
 				jobs:        v.jobs,
 				stop:        make(chan struct{}),
+				stopped:     make(chan struct{}),
 				shutdown:    v.stop,
 			}
 			v.workers = append(v.workers, w)
@@ -227,6 +229,9 @@ func (v *offloadingPostVerifier) scale(target int) {
 		for _, worker := range toStop {
 			close(worker.stop)
 		}
+		for _, worker := range toStop {
+			<-worker.stopped
+		}
 	}
 }
 
@@ -293,6 +298,7 @@ func (v *offloadingPostVerifier) Close() error {
 func (w *postVerifierWorker) start() {
 	w.log.Info("starting")
 	defer w.log.Info("stopped")
+	defer close(w.stopped)
 
 	for {
 		// First try to process a prioritized job.

activation/post_verifier_scaling_test.go

@@ -62,6 +62,7 @@ func TestPostVerifierScaling(t *testing.T) {
 	require.NoError(t, err)
 
 	v.scale(0)
+
 	ctx, cancel = context.WithTimeout(context.Background(), 10*time.Millisecond)
 	defer cancel()
 	err = v.Verify(ctx, &shared.Proof{}, &shared.ProofMetadata{})