Not really. If one believes that there was a worldwide flood and that God cannot lie that belief can be tested. It would fail. There are many examples of how the Bible could be tested and it would fail if one does so rationally. Now if one wants to be irrational then nothing can be tested. But what is nice about reality is that it can be tested. And remember, in a face to face debate it is allowed to slap a solipsist that tries to deny reality. Sometimes nonverbal communication is needed to get a point across.
No, it can't. That is a belief, which is called an assumption and no, you don't have to read these link. I will explain how to test reality below:
Science and the Modern World
Objectivity in science - its uncertainty & limits
http://press-files.anu.edu.au/downloads/press/p98831/pdf/ch0615.pdf
I mean, there is a reason for how come we have the following 2 concepts:
Methodological naturalism versus philosophical naturalism.
So I am going to give you a test about reality: You are going to test what objective reality as existing outside your mind is. You are going once and for all to show philosophy that it is irrational:
https://en.wikipedia.org/wiki/Simulation_hypothesis
So here is what you are going to test. You are not going to rely on models/maps of the landscape as science normally do.
For the case that you are in real objective reality or in a computer simulation reality(note you could also be e.g. a Boltzmann Brain) you are going to move out of your mind and outside reality and observe once and for what objective reality is independent of the mind. Now you have done so:
So Subduction Zone, what reality are we in? The real one, a computer simulation, are you a Boltzmann brain, is God actually real and so on? I mean you can test in effect the metaphysical and ontological status of objective reality. You are so special, that you can solve over 2000+ years of human inquiry. Just write your finding to at least one scientific body and trust me, there is a Nobel prize in it. You will be bigger than Einstein and the rest. You have tested what objective reality really is and observed objective reality independent of your mind.
So here is your problem in your philosophy of science:
You have 3 options:
Nothing can be tested.
Everything can be tested including what objective reality is as independent of the mind.
There is a limit to human rationality and thus we get cognitive relativism. It is in practice possible to believe differently about objective reality as independent of the mind. Thus we get this for those who actually understand science;
https://undsci.berkeley.edu/article/0_0_0/whatisscience_12
Note the last part about supernatural.