Fix "Testing with Rust / Writing Tests" example (#5539)

## 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The harnes.rs code that currently appears in the book seems to be out of
date (it doesn't compile).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](https://github.com/FuelLabs/sway/blob/1fee545237a53423a2c6828da95837d380fed5a6/templates/sway-test-rs/template/tests/harness.rs#L1-L36).

(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](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [ ] I have requested a review from the relevant team or maintainers.

---------

Co-authored-by: IGI-111 <[email protected]>
Co-authored-by: Sophie Dankel <[email protected]>

docs/book/src/testing/testing-with-rust.md

@@ -180,10 +180,13 @@ following:
 ```rust,ignore
 use fuels::{prelude::*, types::ContractId};
 
-// Load ABI from JSON
-abigen!(TestContract, "out/debug/my-fuel-project-abi.json");
+// Load abi from json
+abigen!(Contract(
+    name = "MyContract",
+    abi = "out/debug/my-fuel-project-abi.json"
+));
 
-async fn get_contract_instance() -> (TestContract, ContractId) {
+async fn get_contract_instance() -> (MyContract<WalletUnlocked>, ContractId) {
     // Launch a local network and deploy the contract
     let mut wallets = launch_custom_provider_and_get_wallets(
         WalletsConfig::new(
@@ -192,8 +195,10 @@ async fn get_contract_instance() -> (TestContract, ContractId) {
             Some(1_000_000_000), /* Amount per coin */
         ),
         None,
+        None,
     )
-    .await;
+    .await
+    .unwrap();
     let wallet = wallets.pop().unwrap();
 
     let id = Contract::load_from(
@@ -206,11 +211,11 @@ async fn get_contract_instance() -> (TestContract, ContractId) {
         ),
     )
     .unwrap()
-    .deploy(&wallet, TxParameters::default())
+    .deploy(&wallet, TxPolicies::default())
     .await
     .unwrap();
 
-    let instance = TestContract::new(id.to_string(), wallet);
+    let instance = MyContract::new(id.clone(), wallet);
 
     (instance, id.into())
 }