Add unit tests notes

Rust/Unit Tests.md

@@ -0,0 +1,60 @@
+# Unit Tests
+
+## Test Functions
+
+```rs
+// module code here
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_func() {
+        //ARRANGE
+
+        // ACT
+
+        // ASSERT
+        assert!(bool, "optional custom error message");
+        assert_eq!(expected, actual, "optional custom error message");
+        assert_ne!(expected, actual, "optional custom error message");
+    }
+
+    #[test]
+    #[should_panic("optional custom error message")]
+    fn test_func() {/* ... */}
+
+    #[test]
+    #[ignore]
+    fn test_func() {/* ... */}
+}
+```
+
+## Controlling How Tests Are Run
+
+```sh
+cargo test -- --test-threads=<number>  # run tests in parallel (1 no parallelism)
+cargo test -- --show-output  # show content printed in stdout in each test
+cargo test <test_name>  # run only a specific test
+cargo test <partial_test_name>  # run only specific tests
+cargo test -- --ignored  # run tests annotated with #[ignore]
+cargo test --test  # run all integration tests
+```
+
+## Test Organization
+
+Unit Tests:
+
+- Inside the same module as the tested code
+- annotated with `#[cfg(test)]`
+- can test private code by default
+- import tested code with `super::*`
+- compiled only with `cargo test`
+
+Integration tests:
+
+- inside top level `tests` folder
+- need to import tested code by crate name
+- place util functions in a `mod.rs` to avoid testing them, e.g: `tests/common/mod.rs`
+- compiled only with `cargo test`