test[-1] = 11;
test[4] = 11;
This is undefined behavior.(Meaning anything could have happened). In your case you changed the value of b because they are adjacent in the memory where they are allocated. But you shouldn't rely on it. Because this may blow up your program or results in erroneous code behavior most of the time.
The UB you have is because `Accessing an array index out of bound in undefined behavior."
