eth: fix the flaky test of TestSnapSyncDisabling68 (#33896)

fix the flaky test found in
https://ci.appveyor.com/project/ethereum/go-ethereum/builds/53601688/job/af5ccvufpm9usq39

1. increase the timeout from 3+1s to 15s, and use timer instead of
sleep(in the CI env, it may need more time to sync the 1024 blocks)
2. add `synced.Load()` to ensure the full async chain is finished

Signed-off-by: Delweng <delweng@gmail.com>

eth/sync_test.go

@@ -82,15 +82,18 @@ func testSnapSyncDisabling(t *testing.T, ethVer uint, snapVer uint) {
 	if err := empty.handler.downloader.BeaconSync(full.chain.CurrentBlock(), nil); err != nil {
 		t.Fatal("sync failed:", err)
 	}
-	// Downloader internally has to wait for a timer (3s) to be expired before
-	// exiting. Poll after to determine if sync is disabled.
-	time.Sleep(time.Second * 3)
-	for timeout := time.After(time.Second); ; {
+	// Snap sync and mode switching happen asynchronously, poll for completion.
+	timeout := time.NewTimer(15 * time.Second)
+	tick := time.NewTicker(100 * time.Millisecond)
+	defer timeout.Stop()
+	defer tick.Stop()
+
+	for {
 		select {
-		case <-timeout:
+		case <-timeout.C:
 			t.Fatalf("snap sync not disabled after successful synchronisation")
-		case <-time.After(100 * time.Millisecond):
-			if empty.handler.downloader.ConfigSyncMode() == ethconfig.FullSync {
+		case <-tick.C:
+			if empty.handler.synced.Load() && empty.handler.downloader.ConfigSyncMode() == ethconfig.FullSync {
 				return
 			}
 		}