Good to hear you changed your mind about scientific tests.
Surely this is trivially obvious?
That's a confused non-sequitur. What follows from your first statement is that if God is proven true to someone, God was always true (by 'true' here I assume you mean the existence of God). But if God is proven true to someone, they may have proven it themselves, someone else may have provided them with proof, or - as you suggest - God may have given proof of himself.
However, God is an ill-defined and unfalsifiable concept, so unprovable (as for Russell's Teapot). Strictly, proof is only applicable to analytic (logical, mathematical) statements - although we generally loosen the criteria for trivially obvious states of affairs. But as God is ill-defined and unfalsifiable, then someone who thinks they have proof of, or have proved, God's existence, is either mistaken or deluded (unless they have obtained a well-defined and falsifiable God concept - let's hear it!); so logically, as an unprovable concept, the only certainty is that God might exist; I guess this is why faith is so valued in theism.