Tthe original post: "Could anyone tell where I can find a FREE tool that I may use to test my circuits?"

I post "to test to prove is extremely complicated/impossible"

Andy agrees
"Of course it won't be easy, and the necessary equipment won't be cheap"

Yes, there is a remote possibility that "automated testing" can be designed that will catch ALL variations of the bug I decsribed, but visualize again this expanded version of my example:
Desigan an automated testing that will catch a bug when ANY interrupt happen when ANY other interrupt is executing ANYWHERE in the ISR and that first interrupt happend ANYWHERE in the main and its called funstions.

I am sure that "it won't be easy, and won't be cheap".

So the idea of the OP to get free testing software that is worth sh.. is just an illusion.

so, I repeat "I know of no testing that can prove that something works". If NASA or some entity spend millions of dollars testing something it will, of course, make it much more likely that it works, but as we all know it did not prove that it worked.

Erik