GetStaleTimestamp may return future TSO

[you06]

This test may return future TSO when calling GetStaleTimestamp, which is a correctness risk.

func TestNonFutureStaleTSO(t *testing.T) {
	o := oracles.NewEmptyPDOracle()
	oracles.SetEmptyPDOracleLastTs(o, oracle.GoTimeToTS(time.Now()))
	for i := 0; i < 100; i++ {
		time.Sleep(10 * time.Millisecond)
		now := time.Now()
		upperBound := now.Add(5 * time.Millisecond) // allow 5ms time drift

		closeCh := make(chan struct{})
		go func() {
			time.Sleep(100 * time.Microsecond)
			oracles.SetEmptyPDOracleLastTs(o, oracle.GoTimeToTS(now))
			close(closeCh)
		}()
	CHECK:
		for {
			select {
			case <-closeCh:
				break CHECK
			default:
				ts, err := o.GetStaleTimestamp(context.Background(), oracle.GlobalTxnScope, 0)
				assert.Nil(t, err)
				staleTime := oracle.GetTimeFromTS(ts)
				if staleTime.After(upperBound) && time.Since(now) < time.Millisecond /* only check staleTime within 1ms */ {
					assert.Less(t, staleTime, upperBound, i)
					t.FailNow()
				}
			}
		}
	}
}

The test should pass because stale timestamp should not be a future ts. But we got:

Error:      	"2023-11-09 15:44:50.757 +0900 JST" is not less than "2023-11-09 15:44:50.751395 +0900 JST m=+0.030711501"