docs: add shell completion testing section

[mitre88]

Summary

• add a new Shell Completion section to docs/testing.md
• document a minimal helper using click.shell_completion.ShellComplete to test completion suggestions directly
• add an Unreleased changelog entry in CHANGES.rst

This uses the same testing pattern already used in Click's own shell completion tests and gives users a stable migration target from older get_choices-based examples.

Fixes #2299