set SftpException as cause when listDirectories throws IOException

for issue MULE-7821
While I have used mockito for a lot of things, I have not used it to set a private (or even protected) class member. The purpose of this class is to provide the setter to set the channel which is mocked with Mockito. As you might see in the test, I have mocked the channel to throw an SftpException when channel.ls() method is called (see line 90 and then line 94). If you have a better way to do it, I will defer to you and learn something in the process.