Update the tutorial for testing (#91)

docs/tutorials/testing.mdx

@@ -17,15 +17,14 @@ Contract](write-a-contract.mdx), a simple test will look like this.
 
 ```rust
 #![no_std]
-use soroban_sdk::{contractimpl, vec, Env, Symbol, Vec};
+use soroban_sdk::{contractimpl, symbol, vec, Env, Symbol, Vec};
 
 pub struct Contract;
 
-#[contractimpl(export_if = "export")]
+#[contractimpl]
 impl Contract {
     pub fn hello(env: Env, to: Symbol) -> Vec<Symbol> {
-        const GREETING: Symbol = Symbol::from_str("Hello");
-        vec![&env, GREETING, to]
+        vec![&env, symbol!("Hello"), to]
     }
 }
 ```
@@ -36,26 +35,38 @@ impl Contract {
 ```rust
 #![cfg(test)]
 
-use super::{Contract, hello};
-use soroban_sdk::{vec, Env, FixedBinary};
+use super::{Contract, ContractClient};
+use soroban_sdk::{symbol, vec, BytesN, Env};
 
 #[test]
 fn test() {
     let env = Env::default();
-    let contract_id = FixedBinary::from_array(&env, [0; 32]);
+    let contract_id = BytesN::from_array(&env, &[0; 32]);
     env.register_contract(&contract_id, Contract);
+    let client = ContractClient::new(&env, &contract_id);
 
-    let words = hello::invoke(&env, &contract_id, &Symbol::from_str("Dev"));
+    let words = client.hello(&symbol!("Dev"));
     assert_eq!(
         words,
-        vec![&env, Symbol::from_str("Hello"), Symbol::from_str("Dev"),]
+        vec![&env, symbol!("Hello"), symbol!("Dev"),]
     );
 }
 ```
 
+
 </TabItem>
 </Tabs>
 
+:::info
+The above example is a [Rust unit test] that lives inside the `src/` directory.
+Note that if you place the test in the `tests/` directory it becomes a [Rust
+integration test] with the test being compiled separately. Integration tests
+require `#![cfg(feature = "testutils")]` at the top of the file and to be run
+with the `testutils` feature enabled, e.g. `cargo test --features testutils`, to
+enable the generated Soroban test utilities.
+:::
+
+
 In any test the first thing that is always required is an `Env`, which is the
 Soroban environment that the contract will run inside of.
 
@@ -69,23 +80,26 @@ if the test will deploy the contract multiple times, or deploy multiple
 contracts, each should use their own IDs.
 
 ```rust
-let contract_id = FixedBinary::from_array(&env, [0; 32]);
-env.register_contract(&contract_id, HelloContract);
+let contract_id = BytesN::from_array(&env, &[0; 32]);
+env.register_contract(&contract_id, Contract);
 ```
 
 All public functions within an `impl` block that is annotated with the
-`#[contractimpl]` attribute have an `invoke` function generated, that can be
-used to invoke the contract function within the environment.
+`#[contractimpl]` attribute have a corresponding function generated in a
+generated client type. The client type will be named the same as the contract
+type with `Client` appended. For example, in our contract the contract type is
+`Contract`, and the client is named `ContractClient`.
 
 ```rust
-let words = hello::invoke(&env, &contract_id, &Symbol::from_str("SourBun"));
+let client = ContractClient::new(&env, &contract_id);
+let words = client.hello(&symbol!("Dev"));
 ```
 
 The values returned by functions can be asserted on:
 ```rust
 assert_eq!(
     words,
-    vec![&env, Symbol::from_str("Hello"), Symbol::from_str("SourBun"),]
+    vec![&env, symbol!("Hello"), symbol!("Dev"),]
 );
 ```
 
@@ -103,3 +117,6 @@ test test::test ... ok
 ```
 
 Try changing the values in the test to see how it works.
+
+[Rust unit test]: https://doc.rust-lang.org/rust-by-example/testing/unit_testing.html
+[Rust integration test]: https://doc.rust-lang.org/rust-by-example/testing/integration_testing.html