Create TESTING_GUIDE.md

code/TESTING_GUIDE.md

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+```markdown
+# Malachite code testing guide
+```
+This short guide explains where to start when you want to run or extend tests
+in the `code/` part of the repository.
+
+## 1. Recommended commands
+
+From the repository root:
+
+- `cargo test` – run the default Rust test suite.
+- `cargo test -p <crate-name>` – run tests for a single crate when focusing on a specific module.
+- `cargo test -- --nocapture` – see test output when debugging failing cases.
+
+Check the main README and any crate-level `README.md` files for additional, more targeted commands.
+
+## 2. Working with examples
+
+Some features are easier to exercise via example applications and tutorials
+linked from `ARCHITECTURE.md` and `docs/tutorials/`. When you touch consensus-critical
+code, try to:
+
+- add or update an example that demonstrates the new behaviour, and
+- add at least one unit or integration test that guards against regressions.
+
+## 3. Before opening a PR
+
+- Make sure the relevant tests pass locally.
+- Prefer small, well-named tests that document behaviour.
+- When fixing a bug, include a test that fails before the fix and passes after it.