cluster: make TestStoreOverloadedWithReplace stable (#8068)

ref #7969

Signed-off-by: husharp <jinhao.hu@pingcap.com>

Co-authored-by: ti-chi-bot[bot] <108142056+ti-chi-bot[bot]@users.noreply.github.com>

server/cluster/cluster_test.go

@@ -3480,10 +3480,11 @@ func TestStoreOverloadedWithReplace(t *testing.T) {
 	re.False(oc.AddOperator(op3))
 	ops, _ := lb.Schedule(tc, false /* dryRun */)
 	re.Empty(ops)
-	// sleep 2 seconds to make sure that token is filled up
-	time.Sleep(2 * time.Second)
-	ops, _ = lb.Schedule(tc, false /* dryRun */)
-	re.NotEmpty(ops)
+	// make sure that token is filled up
+	testutil.Eventually(re, func() bool {
+		ops, _ = lb.Schedule(tc, false /* dryRun */)
+		return len(ops) != 0
+	})
 }
 
 func TestDownStoreLimit(t *testing.T) {