Add changeset

Author: samtstern

.changeset/cold-bats-hammer.md

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+---
+'@firebase/rules-unit-testing': minor
+---
+
+Release `@firebase/rules-unit-testing` to replace the `@firebase/testing` package. The new
+package is API compatible but has the following breaking behavior changes:
+
+  * `assertFails()` will now only fail on `PERMISSION DENIED` errors, not any error.
+  * `initializeAdminApp()` now relies on `firebase-admin` rather than imitating the Admin SDK.