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....
I suspect @speeter will have long disappeared, as that post was a year ago :)
Just a thought: have a look at (the first part of) https://raspberrypi.stackexchange.com/a/61086, which claims it's a generic Linux problem rather than just a RPi one. It's not quite the same messages, but worth investigating?