cache: deflake TestCacheCompactionResync

The test had a race condition where Phase 3 checked lastStartRev
before the cache finished updating it. The mw.registered channel
was already closed from Phase 1, so <-mw.registered returned
immediately instead of waiting.

Fix: Add resetRegistered() to create a fresh channel for Phase 3,
ensuring the test properly waits for the cache to complete recovery.

Fixes #21150

Signed-off-by: HossamSaberX <hossam.sabeer55@gmail.com>

cache/cache_test.go

@@ -455,7 +455,9 @@ func TestCacheCompactionResync(t *testing.T) {
 	}
 
 	t.Log("Phase 3: resync after compaction")
+	mw.resetRegistered()
 	mw.triggerCreatedNotify()
+	<-mw.registered
 	expectSnapshotRev := int64(20)
 	ctxResync, cancelResync := context.WithTimeout(t.Context(), time.Second)
 	defer cancelResync()
@@ -558,6 +560,8 @@ func (m *mockWatcher) getLastStartRev() int64 {
 }
 
 func (m *mockWatcher) signalRegistration() {
+	m.mu.Lock()
+	defer m.mu.Unlock()
 	select {
 	case <-m.registered:
 	default:
@@ -565,6 +569,12 @@ func (m *mockWatcher) signalRegistration() {
 	}
 }
 
+func (m *mockWatcher) resetRegistered() {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	m.registered = make(chan struct{})
+}
+
 func (m *mockWatcher) streamResponses(ctx context.Context, out chan<- clientv3.WatchResponse) {
 	defer func() {
 		close(out)