docs: document test layers, shared course fixtures, and CI expectations

[erseco]

Summary

Rewrites the ## Testing section of docs/development.md to give contributors a current contract for the test suite. It documents the unit-vs-integration split with exact commands, the shared create_temporary_course factory and course_creation_lock fixture consolidated in #62 (including why course creation is serialized across pytest-xdist workers), a forward reference to the HTML-fixture regression scaffolding from #68, and CI expectations cross-referenced to the README (single source of truth for version numbers).

Linked issue

Closes #67

Specification (SDD)

Update only docs/development.md's Testing section to document: (a) the unit (tests/unit/, no Moodle) vs integration (tests/test_*.py, --integration + live Moodle) layers with the exact commands (make test-unit / pytest tests/unit / make test-local / pytest --integration --moodle-env local -m integration -n auto); (b) the shared create_temporary_course(session, base_url, sesskey, *, prefix, **kwargs) factory and course_creation_lock fixture from tests/conftest.py, why creation is serialized (Moodle-side mdl_context unique-constraint race, cross-process file lock), and that new integration modules needing a temp course must use the factory instead of calling create_course() directly; (c) a short forward reference to the HTML-fixture regression tests; (d) CI expectations by cross-reference, keeping authoritative Python/Moodle numbers in the README. Public contract: every documented command must actually work.

TDD evidence

No automated test is added because this change is documentation-only (issue section 6 states classic red/green does not apply). Verification per the issue's adapted test plan:

• Unit layer still green: pytest tests/unit -q -> all tests passed (..... 100%, no failures).
• Docs build: python -m mkdocs build completed ("Documentation built in 1.55 seconds"). The only warnings are pre-existing and unrelated to this change (the auto-generated, git-ignored cli.md referenced by nav/quickstart); no new warning references development.md.
• Every referenced name verified to exist: create_temporary_course, course_creation_lock, and _course_creation_lock in tests/conftest.py; make test-unit, make test-local, make test-staging, make test, make docs in the Makefile; the CI matrix in README.md's Testing section.

Test plan

• Added/authored failing test(s) first (or: docs-only, N/A). Docs-only, N/A.
• Confirmed red before implementing. Docs-only, N/A.
• Implemented minimal change.
• Focused tests green.
• Full "pytest tests/unit" green.

Commands run:

pytest tests/unit -q
PYTHONPATH=src python -m mkdocs build

Backwards compatibility

No behavior change. Documentation only; no source, test, or fixture files touched.

Documentation

docs/development.md only: the ## Testing section was restructured to add a "Test Layers" overview, a "Shared Course-Creation Fixtures" subsection (with a "Why course creation is serialized" explanation and a mandatory-use note), an "HTML-Fixture Regression Tests" forward reference to #68, and a "CI Expectations" subsection that cross-references the README rather than duplicating version numbers. The example test now uses the shared create_temporary_course factory.

Notes for reviewers

I stayed strictly within my file-ownership boundary: the only file changed is docs/development.md (confirmed via git status/git diff --stat). No source file, no test file, no tests/conftest.py, no README.md, no docs/recipes.md, and no other docs were touched. Fixture and command names were verified against the live tests/conftest.py and Makefile. The #68 reference is intentionally a forward reference (that sibling PR owns the actual scaffolding); this PR does not duplicate its content.