Updated the pytest to support strict return values detection

[fumedev]

CHANGED

• Created a new test file testing/test_return_values.py in the testing directory of the pytest codebase. Added test cases to ensure that when a test function returns a value other than None, pytest raises a warning or fails the test.
• Modified the existing check in test collection and execution logic by adding a new command-line option --strict-return-values which, when enabled, will activate the new behavior of failing the test if a non-None value is returned.
• Added a new warning class ReturnTestWarning to src/_pytest/warnings.py to represent the specific warning raised when a test function returns a non-None value.
• Updated the documentation in doc/en/warnings.rst to include the new warning class and detail the new command-line option, explaining its purpose and when to use it.