Add a Testing chapter for backend packages

[jensens]

Why

Plone 6 has no backend testing documentation. The Developer guide has no Testing section (only Volto testing, under Contributing), and the only rendered HTML for plone.testing and plone.app.testing lives on the legacy 5.docs.plone.org domain.

@davisagli sketched a full outline for such a chapter in a comment on plone/pytest-plone#16 about two years ago, covering Volto/Jest/Cypress, unittest/zope.testrunner, pytest, Robot, and testbrowser. This PR picks up the two pieces of that outline that cover backend Python testing and deliberately leaves the rest to build on top. Please treat the structure as a starting point — I expect there are opinions on placement, and I am happy to move things.

What

Five pages, Diataxis-classified:

Explanation — conceptual-guides/testing.md
: The kinds of tests (unit / integration / functional / acceptance), why Plone has testing layers at all, and how to choose between the two runners.

How-to — developer-guide/testing/
: - write-a-testing-layer.md — the keystone, placed first: writing the testing.py layer both runners share. Shows the declarative PloneWithPackageLayer form for simple add-ons and the PloneSandboxLayer subclass form for complex ones.
: - zope-testrunner.md — write and run tests with unittest and zope.testrunner.
: - pytest.md — write and run tests with pytest, linking to the published pytest-plone documentation.

Scope

• Backend Python only. Volto testing lives in the Volto repository and is cross-referenced, not duplicated.
• The deep API reference for the layer machinery stays in plone.app.testing / plone.testing; the how-to links there rather than reproducing it.
• A follow-up could modernize and host those two package docs, then link the hosted versions from here.

Verification

• Every zope-testrunner command shown was executed against a real package. The two layer forms are taken from real coredev packages (plone.app.dexterity, plone.volto) and the plone.app.testing README, with every import checked against the installed packages.
• Vale reports 0 errors, 0 warnings, 0 suggestions on the new pages; the vocabulary additions it needed are included.
• make html produces no new warnings, and linkcheck finds no broken links on the new pages.

Related

Companion to the newly published pytest-plone documentation (plone/pytest-plone#56), which this chapter's pytest guide links to.

[gforcada]

Nice job! 🌟

[jensens]

Thanks for the review, @gforcada! All five addressed in the last commit:

• --test-path vs --auto-path — you're right, and thanks for catching it. My --auto-path lead came from buildout.coredev's run-tests.sh, not the common case. The primary command is now zope-testrunner --test-path src -s my.addon (the form the plone.meta tox environments generate), with --auto-path kept as the pip-installed-in-dev-mode alternative. Verified both run.
• Cypress for Volto — agreed the frontend tooling is in flux (Cypress → Playwright, Aurora, …). I dropped the specific tool and defer to the Volto documentation instead, so this backend page does not go stale on a frontend decision. Happy to add a concrete link if there's a canonical Volto testing page to point at.
• pytest --cov — added a pip install pytest-cov note.
• Vocabulary sorting — sorted the additions into place, and trimmed them: the class names only ever appear in code spans, which Vale does not check, so they were never needed. Down to the terms that actually appear in prose.
• Backtick pytest — applied your suggestion.

Vale is back to 0/0/0 on the five pages.