Add `waitForRouter` test helper

What does this MR do and why?

Adds a helper to conditionally call router.onReady or router.isReady based on the active Vue version.

https://gitlab.slack.com/archives/C02719N90MP/p1728014002979929

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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How to set up and validate locally

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Related to #497749 (closed)