[DSLX] Create example where we use assert_eq! in a function called by a test

When running tests through DSL interpretation is clear what `assert_eq!`'s behavior is. Hypothetically they could be synthesized as well by desugaring to `fail!`. I do not believe there's currently a test in the suite that demonstrates `assert_eq!` usage in the body of a function that's not decorated by `#![test]` (i.e. in a function that is *called* by a `#![test]`), so we should make one and corresponding documentation on what users can expect in doing this, even in functions that are not only-called-from-test-entry-points.