Why do you think python (or anything else) is going to open any terminal window? And what OS are you on anyway?
For QProcess:setProgram you should have process.setProgram("python");, only. But in this case I think it's getting overridden by your process.start("python test.py"); so it doesn't matter, but it's wrong.
If you want to grab a sub-process's output, you have to handle QProcess::readyReadStandardOutput(), and in there something like QByteArray QProcess::readAllStandardOutput(), if that's what you mean by "How can I get the 'Hello' output?".
How this relates to Trying to implement thumbnail list I have no idea....