Update rand_big() precondition

[zjma]

Description

Update the invariant of a test-only function rand_big.

What is the change being pushed?

rand_big() was commented to take a non-negative x as input,
while it should assume x is a positive integer.
Calling this out in the function comment.

Why are you pushing this change?

The current comment is misleading.

How is this implemented?

Comments are updated.

Type of Change

Tiny refactoring.

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I identified and added all keyless stakeholders and component owners affected by this change as reviewers
• I tested both happy and unhappy path of the functionality
• I have made corresponding changes to the documentation