Fix "Testing with Rust / Writing Tests" example

[smoelius]

Description

The harnes.rs code that currently appears in the book seems to be out of date (it doesn't compile).

The proposed code was copied from the sway-test-rs template.

(The Sway tutorial is very good, BTW.)

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[smoelius]

I'm not sure if the checklist items apply. I think this PR is good to go.

[sarahschwartz]

A more permanent solution for this would be to import the code snippet from a tested example file. You can see how that is done here: https://github.com/FuelLabs/sway/blob/master/docs/book/src/basics/converting_types.md