gh-101100: Define test.regrtest module to fix references

[hugovk]

Fixes 7 Sphinx warnings:

Doc/library/test.rst:18: WARNING: py:mod reference target not found: test.regrtest Doc/library/test.rst:18: WARNING: py:mod reference target not found: test.regrtest Doc/library/test.rst:88: WARNING: py:mod reference target not found: test.regrtest Doc/library/test.rst:162: WARNING: py:mod reference target not found: test.regrtest Doc/library/test.rst:230: WARNING: py:mod reference target not found: test.regrtest Doc/library/test.rst:463: WARNING: py:mod reference target not found: test.regrtest Doc/library/test.rst:480: WARNING: py:mod reference target not found: test.regrtest 

• Issue: Fix all Sphinx reference warnings in the documentation #101100

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📚 Documentation preview 📚: https://cpython-previews--112381.org.readthedocs.build/

[miss-islington-app]

Thanks @hugovk for the PR 🌮🎉.. I'm working now to backport this PR to: 3.11, 3.12.
🐍🍒⛏🤖 I'm not a witch! I'm not a witch!

[bedevere-app]

GH-112390 is a backport of this pull request to the 3.12 branch.

[bedevere-app]

GH-112391 is a backport of this pull request to the 3.11 branch.