docs(mosaic): add the golden rule of assertions to the testing guide (#10441)

Author: alexcarpenter

.changeset/mosaic-testing-golden-rule.md

@@ -0,0 +1,2 @@
+---
+---

packages/mosaic/docs/testing.md

@@ -8,6 +8,12 @@ We follow [Write tests. Not too many. Mostly integration.](https://kentcdodds.co
   a user. Past that point, tests cost more than they return.
 - **Don't test implementation details.** A test that breaks on a refactor that
   keeps the behavior is testing the wrong thing.
+- **A test fails if, and only if, the feature stops doing what it is for.**
+  Before writing or reviewing a test, ask "when will this fail?". If the answer
+  includes a refactor that keeps the behavior, a renamed helper, or something
+  the test doesn't control, such as real network or timing, the test is wrong.
+  See
+  [The Golden Rule of Assertions](https://www.epicweb.dev/the-golden-rule-of-assertions).
 - **Static checks come first.** TypeScript and ESLint catch typos and shape
   errors, so tests don't need to.