docs: add README for Frappe test utilities (#28016)

frappe/tests/README.md

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+# Frappe Test Utilities
+
+This README provides an overview of the test utilities available in the Frappe framework, particularly focusing on the `frappe/tests/utils.py` file. These utilities are designed to facilitate efficient and effective testing of Frappe applications.
+
+## Main Functions
+
+The `utils.py` file contains several key components:
+
+1. Test record generation utilities
+2. Test case classes (UnitTestCase and IntegrationTestCase)
+3. Context managers for various testing scenarios
+4. Utility functions and decorators
+
+## Test Case Classes
+
+### UnitTestCase
+
+This class extends `unittest.TestCase` and provides additional utilities specific to Frappe framework. It's designed for testing individual components or functions in isolation.
+
+Some key methods and features include:
+
+- Custom assertions (e.g., `assertQueryEqual`, `assertDocumentEqual`)
+- HTML and SQL normalization
+- Context managers for user switching and time freezing
+
+### IntegrationTestCase
+
+This class extends `UnitTestCase` and is designed for integration testing. It provides features for:
+
+- Automatic database setup and teardown
+- Database connection management
+- Query counting and Redis call monitoring
+- Lazy loading of test record dependencies
+
+For a complete list of methods and their usage, please refer to the actual code in `frappe/tests/utils.py`.
+
+## Usage
+
+To use these test utilities in your Frappe application tests, you can inherit from the appropriate test case class:
+
+```python
+from frappe.tests.utils import UnitTestCase
+
+class MyTestCase(UnitTestCase):
+    def test_something(self):
+        # Your test code here
+        pass
+```
+
+Remember that this README provides an overview as of the time of writing. Always refer to the actual code for the most up-to-date and detailed information on available methods and their usage.
+
+## Contributing
+
+If you're adding new test utilities or modifying existing ones, please ensure to update this README accordingly.