docs: add test readability guidance

tests/write-tests.md

@@ -133,6 +133,49 @@ package? Below are a few examples:
   user can easily understand why it failed by providing a useful error
   message.
 
+### Write tests that are easy to review
+
+Clear tests help reviewers and future contributors understand what behavior is
+being checked and why that behavior matters. As you write tests, try to make the
+main idea of each test visible without requiring readers to reverse-engineer the
+setup.
+
+- **Use descriptive test names:** Name each test after the behavior, condition,
+  or edge case it checks. For example, `test_add_numbers_accepts_negative_values`
+  tells readers more than `test_add_numbers_2`.
+- **Keep one main behavior in focus:** A test can contain several assertions, but
+  they should support one clear idea. If a test starts checking several unrelated
+  behaviors, split it into smaller tests.
+- **Add comments only where they help:** A short comment is useful when a
+  fixture, regression, or unusual edge case is not obvious from the assertion.
+  Avoid comments that repeat the code.
+- **Keep setup, action, and assertion easy to scan:** Arrange the test so readers
+  can quickly see what input is prepared, what code is run, and what result is
+  expected.
+
+For example, a test based on the `add_numbers` function in the
+[pyOpenSci Python package template](https://github.com/pyOpenSci/pyos-package-template)
+can make its inputs, action, and expected result visible at a glance:
+
+```python
+from my_package.example import add_numbers
+
+
+def test_add_numbers_returns_sum():
+    first_number = 1
+    second_number = 2
+    expected_sum = 3
+
+    actual_sum = add_numbers(first_number, second_number)
+
+    assert actual_sum == expected_sum
+```
+
+For more guidance on structuring readable tests, see pytest's
+[anatomy of a test](https://docs.pytest.org/en/stable/explanation/anatomy.html),
+which explains the arrange, act, assert, and cleanup phases, and the package
+template's [example unit test](https://github.com/pyOpenSci/pyos-package-template/blob/main/template/%7B%25%20if%20use_test%20%25%7Dtests%7B%25%20endif%20%25%7D/unit/test_example.py.jinja).
+
 ## Next steps
 
 Now that you understand what and why to test, explore the [three types of