docs(testing): await async-gated content in CV tests

[racitores]

Summary

• Add CV guidance for the flake class where a container mounts before a child value settles (own query / debounce / skeleton).
• Encode it as Golden Rule 13, a writing-tests pitfall, self-review checklist item, diagnosing-failures row, assertion pattern, and What NOT to Do example in mobile-testing component-view refs.
• Motivated by the intermittent PredictMarketDetails Positions-tab $60 failure on metamask-mobile main CI (sync getByText after awaiting only the tab container).

Test plan

• yarn audit:skills — 0 errors
• yarn test — 61/61 pass
• Reviewer: confirm the rule complements (does not duplicate) the existing stale-press-target guidance from docs(testing): document CV flakiness and high-leverage assert patterns #109 / metamask-mobile #34241

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