TestFromClientSide5.testAppendWithoutWAL is flaky

Not sure whether it is also flaky on branch-2.x, but at least on master, when we call Table.getScanner and get the returned ResultScanner, it does not mean that we have finished sending the request to region server, as the actual implementation is asynchronous. If we want a stable result, we need to at least get one row from the scanner.