docs(tests): add instructions for cleanup

Author: rolandszoke

__tests__/Clock.test.jsx

@@ -12,6 +12,10 @@ describe('Clock App', () => {
   );
 
   const clearIntervalSpy = sinon.spy(global, 'clearInterval');
+  /*
+    Please keep the cleanup in a separate afterAll.
+    Otherwise, it will lose the scope of the document, and it won't be able to clear the body.
+  */
   afterAll(cleanup);
   afterAll(() => {
     clock.restore();

__tests__/README.md

@@ -1,3 +1,24 @@
 ## React Testing Library
 
 Heads-up: Currently we are using `@testing-library/react/pure` instead of `@testing-library/react` since our tests are not [isolated](https://kentcdodds.com/blog/test-isolation-with-react).
+
+On writing new tests, please keep the `cleanup` functions in __separate__ repeating or one-time functions like `afterEach` or `afterAll`.
+Otherwise, it will lose the scope of the `document`, and it won't be able to clear the `body`.
+
+BAD
+```js
+  afterAll(() => {
+    cleanup();
+    clock.restore();
+    clearIntervalSpy.restore();
+  });
+```
+
+GOOD
+```js
+  afterAll(cleanup);
+  afterAll(() => {
+    clock.restore();
+    clearIntervalSpy.restore();
+  });
+```

__tests__/router.test.jsx

@@ -15,6 +15,10 @@ import {
 } from 'react-router-dom';
 
 describe('withRouter interaction', () => {
+  /*
+    Please keep the cleanup in a separate afterEach.
+    Otherwise, it will lose the scope of the document, and it won't be able to clear the body.
+  */
   afterEach(cleanup);
   afterEach(() => {
     window.history.replaceState({}, '', '/');