docs: explain filesystem tests in frozen applications

[Lem0nTea2002]

Summary

• Explain why tests stored only inside a PyInstaller PYZ archive are not visible to pytest's filesystem-based collectors.
• Show how to bundle test sources as filesystem data with --add-data.
• Resolve the bundled test directory relative to __file__ and pass that path to pytest.main().

Why

--pyargs can resolve importable packages, but pytest still needs filesystem paths when collecting Python test modules. The documentation now makes that boundary explicit and gives frozen applications a supported filesystem-backed layout.

Related to #14771.

Validation

• uvx tox -e docs -- --show-traceback (256 documentation sources, warnings treated as errors)
• targeted pre-commit hooks for both changed files
• uvx towncrier build --draft --version 9.2.0
• git diff --check

AI assistance

OpenAI Codex assisted with drafting the documentation change and running validation. The human contributor reviewed the complete diff and validation results before this draft pull request was opened and takes responsibility for the contribution.