test(stdlibs/io): add additional test (gnolang#2898)

# Description

Fixed a failing test in the io package. Additionally, I activated some
previously commented-out tests by bypassing the need for `os.CreateTemp`
using `bytes.Buffer`.

<details><summary>Contributors' checklist...</summary>

- [ ] 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
- [ ] 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 Bazalgette <[email protected]>