Expand the testing domain (currently 1 file) into full AL test-authoring knowledge + test skills

[JesperSchulz]

Context

microsoft/knowledge/testing/ contains a single file (transactionmodel-attribute-governs-test-transactions). There is no test-authoring or test-review skill. Testing is one of the highest-value areas for enabling cheap models to ship working BC apps.

Why it matters (admission test)

LLMs routinely get the BC test toolkit wrong — handler functions, test isolation, the test libraries, and TestPage vs. real pages are all BC-specific mechanics that generic training data gets backwards.

Scope / checklist

Knowledge files

• [Test] method structure + GIVEN-WHEN-THEN naming/sectioning.
• [HandlerFunctions] and handler types (Message, Confirm, Page, ModalPage, Report, Request, StrMenu, Send).
• Test isolation & [TransactionModel]; Initialize() re-setup pattern.
• Test libraries — LibraryRandom, LibraryUtility, LibraryLowerPermissions, domain libraries (e.g. LibrarySales), the Assert codeunit.
• TestPage interaction vs. invoking real pages.
• [TestPermissions] / permission testing.
• Test codeunit Subtype = Test, test-runner ranges, mocking via subscribers.

Skills

• microsoft/skills/review/al-test-review.md leaf, wired into al-code-review sub-skills.
• microsoft/skills/author/al-test-generate.md (depends on the generation contract in Add the "develop" half: generation output contract, scaffolding skills, golden templates, and anti-hallucination reference #34).

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Each new knowledge file must pass the README admission test and the CI-validated frontmatter schema.