[fix] stale testing.md link (flutter#8120)

The link didn't seem to make the wiki migration:


![image](https://github.com/user-attachments/assets/24306222-276d-4903-aba4-5638aea0023c)


---

- [x] I’ve reviewed the contributor guide and applied the relevant
portions to this PR.

<details>
  <summary>Contribution guidelines:</summary><br>

- See our [contributor
guide]([https://github.com/dart-lang/sdk/blob/main/CONTRIBUTING.md](https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview)
for general expectations for PRs.
- Larger or significant changes should be discussed in an issue before
creating a PR.
- Dart contributions to our repos should follow the [Dart style
guide](https://dart.dev/guides/language/effective-dart) and use `dart
format`.
- Java and Kotlin contributions should strive to follow Java and Kotlin
best practices
([discussion](flutter#8098)).
</details>

Author: pq

docs/Release-testing.md

@@ -1,2 +1,2 @@
 The following use cases should be tested on all new releases:
-https://github.com/flutter/flutter-intellij/blob/master/testing.md
+[https://github.com/flutter/flutter-intellij/blob/master/testing.md](https://github.com/flutter/flutter-intellij/blob/master/docs/testing.md)