gh-1463: Added Refleak Checker Option under testing.

[Lincoln-developer]

This pull request adds documentation for the -R flag in the python -m test command, enabling reference leak checking. This feature helps developers identify memory leaks when modifying C code.

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📚 Documentation preview 📚: https://cpython-devguide--1468.org.readthedocs.build/

[encukou]

Thank you for the addition!
When first learning about -R, you aren't likely to know what values to use for <warmups> and <repeats>. So, I think it would be good if the first paragraph, and the example, used the defaults, which are specified (somewhat unusually) with a lone colon after the -R: python -m test <test_name> -R :

A follow-up paragraph can then mention specifying <warmups> and <repeats> explicitly (which would help explain what the : is for).

[Lincoln-developer]

Thank you for the addition! When first learning about -R, you aren't likely to know what values to use for <warmups> and <repeats>. So, I think it would be good if the first paragraph, and the example, used the defaults, which are specified (somewhat unusually) with a lone colon after the -R: python -m test <test_name> -R :

A follow-up paragraph can then mention specifying <warmups> and <repeats> explicitly (which would help explain what the : is for).

Thank you @encukou for the insight sighted out, let me work on these changes to have a more clear view of this context.