[test] Consider adding a random increment function to testutils

should we make this part of the testutil? `random` increment?

_Originally posted by @tjaychen in https://github.com/lowRISC/opentitan/pull/14896#discussion_r973490391_

Start by reviewing the testutil context and the discussion in pull request #14896, especially the referenced comment. Determine whether a reusable random-increment helper belongs in testutil and define its expected behavior before making a change; no file or test path is identified in the issue.

Written by the indexing model from the issue text.