test(p/uint256): Increase Test Coverage for uint256 Package (#2931)

# Description

<img width="728" alt="Screenshot 2024-10-10 at 2 53 28 PM"
src="https://github.com/user-attachments/assets/0c38769d-b80d-4850-b8c9-6bd3c37d8b6f">

Increased the test coverage of the `p/demo/uint256` package. Previously,
only about 40% was covered, but now it has increased to 90% (checked in
go).

The existing implementation of the uint256 function is unmodified except
for modifying it to use the strconv package.

- [X] Added new tests, or not needed, or not feasible
- [ ] Provided an example (e.g. screenshot) to aid review or the PR is
self-explanatory
- [ ] Updated the official documentation or not needed
- [ ] No breaking changes were made, or a `BREAKING CHANGE: xxx` message
was included in the description
- [X] Added references to related issues and PRs
- [ ] Provided any useful hints for running manual tests
- [ ] Added new benchmarks to [generated
graphs](https://gnoland.github.io/benchmarks), if any. More info
[here](https://github.com/gnolang/gno/blob/master/.benchmarks/README.md).
</details>

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Co-authored-by: Morgan <[email protected]>