Add test guidelines

Author: chrismccord

usage-rules/elixir.md

@@ -41,3 +41,15 @@
 - Read the docs and options before using tasks (by using `mix help task_name`)
 - To debug test failures, run tests in a specific file with `mix test test/my_test.exs` or run all previously failed tests with `mix test --failed`
 - `mix deps.clean --all` is **almost never needed**. **Avoid** using it unless you have good reason
+
+## Test guidelines
+
+- **Always use `start_supervised!/1`** to start processes in tests as it guarantees cleanup between tests
+- **Avoid** `Process.sleep/1` and `Process.alive?/1` in tests
+  - Instead of sleeping to wait for a process to finish, **always** use `Process.monitor/1` and assert on the DOWN message:
+
+      ref = Process.monitor(pid)
+      assert_receive {:DOWN, ^ref, :process, ^pid, :normal}
+
+   - Instead of sleeping to synchronize before the next call, **always** use `_ = :sys.get_state/1` to ensure the process has handled prior messages
+