Document arrange-act-assert testing pattern

[philprime]

📜 Description

This PR introduces a new Cursor rule file, .cursor/rules/testing.md, which provides comprehensive guidelines for testing within the Sentry Cocoa SDK. The rule outlines standard test structure, emphasizes the Arrange-Act-Assert (AAA) pattern, details fixture usage, and covers naming conventions based on an analysis of existing test files.

💡 Motivation and Context

The primary motivation is to standardize testing practices and provide a clear guide for developers, especially new contributors, on how to structure and write tests. By documenting the prevalent AAA pattern and other common test idioms found in the codebase, this rule aims to improve consistency, readability, and maintainability of tests across the project.

💚 How did you test it?

This change introduces a documentation file (a Cursor rule) based on an analysis of existing test files in the Sentry Cocoa SDK. The content was derived by observing common patterns and best practices already present in the codebase. No code changes were made that require automated tests.

📝 Checklist

You have to check all boxes before merging:

• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed. (This is a new documentation file)
• I updated the wizard if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
_{Learn more about Cursor Agents}

[github-actions]

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## Unreleased

- Document arrange-act-assert testing pattern ([#6141](https://github.com/getsentry/sentry-cocoa/pull/6141))

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