We want it to write to different destination when run fleetctl and unit_test.
One way is to redirect stderr. I could make it happen just around `Check`
```
...
disableStderr()
Check()
enableStderr()
...
```
The other way is to make `Check` write to somewhere, which implements `io.Writer`, else. I use `log.Logger` here because it provides `printf`, but I could see whether or not there exists some other types.