test(media): validate check result messages - #5480

The caption and no-autoplay-audio tests currently call check.evaluate directly, so they still pass if an incomplete diagnostic message is missing. Route their existing cases through testutils.getCheckEvaluate with message verification enabled and explicit mock contexts.

Preserve all 17 existing cases, including incomplete results, caption's none-rule semantics, composed-tree captions, media preloading, allowed duration, looping, and controls.

Part of #5375, following the agreed split into focused migration PRs. Additional check families remain.

Validation:

• Build, repository-wide formatting, ESLint, and TypeScript passed with Node 24.21.0.
• All 23 media tests passed in Chrome 155, including all 17 migrated cases.
• All eight test:unit:* browser suites passed.
• Temporary mutation: removing both incomplete messages leaves the original tests passing (17/17), while the migrated tests detect all four affected cases (13 pass, four expected failures). All mutations were restored.
• Independent diff review: no findings.

The local pnpm test wrapper passed TypeScript but could not spawn the cached pnpm.cjs (EACCES). The browser suites passed using npm-run-all's supported --npm-path option pointing to the same installed pnpm 11.17.0 module. This workaround changes no project files.

Prepared with Codex. Human review pending.