Small tweaks to the unit testing how-to guide (#1095)

* Small tweaks to the unit testing how-to guide

* tweaks

* tweak

* tweaks

* tweaks

* link to example

* tweak

Author: leighmcculloch

docs/build/guides/testing/README.mdx

@@ -5,4 +5,4 @@ sidebar_class_name: sidebar-category-items-hidden
 sidebar_position: 110
 ---
 
-Smart contract testing is vital to ensure safe, resilient, and accurate computation.
+Testing is vital to ensure that smart contracts are safe, resilient, and accurate.

docs/build/guides/testing/basic-contract-tests.mdx

@@ -0,0 +1,50 @@
+---
+title: Unit Tests
+hide_table_of_contents: true
+description: Unit tests are small tests that test smart contracts.
+sidebar_position: 1
+---
+
+Unit tests are small tests that test one piece of functionality within a contract.
+
+The following is an example of a unit test, written to test the [increment contract]. The contract has an `increment` function, that increases a counter value by one on every invocation. The following test invokes that contract's function several times, and checks that the value increases by one.
+
+```rust
+#![cfg(test)]
+use crate::{IncrementContract, IncrementContractClient};
+use soroban_sdk::Env;
+
+#[test]
+fn test() {
+    let env = Env::default();
+    let contract_id = env.register(IncrementContract, ());
+    let client = IncrementContractClient::new(&env, &contract_id);
+
+    assert_eq!(client.increment(), 1);
+    assert_eq!(client.increment(), 2);
+    assert_eq!(client.increment(), 3);
+}
+```
+Ref: https://github.com/stellar/soroban-examples/blob/main/increment/src/test.rs
+
+:::tip
+
+Tests are written in Rust, alongside the contract. Same tools. Same APIs. Same SDK. No context switching! Use your favorite Rust tools and libraries that you'd use for any Rust test.
+
+:::
+
+:::info
+
+The Env created at the beginning of the test is not a simulation of the Soroban Environment. It's the same Soroban Environment that mainnet uses to execute contracts. There are only some minor differences: test utilities are enabled, and the storage backend is different.
+
+:::
+
+It's a simple test, but it's a complete test. There's a full environment setup, used, and torn down in the test, and it happens fast. The Rust test harness runs all the tests for a contract in parallel and each will have its own isolated contract environment.
+
+Most tests, even integration tests and fuzz tests, will look very similar to this unit test. They'll do four things:
+1. Create an environment, the `Env`.
+2. Register the contract(s) to be tested.
+3. Invoke functions using a client.
+4. Assert the outcome.
+
+[increment contract]: https://github.com/stellar/soroban-examples/blob/main/increment/src/lib.rs