test(performance-timer): reduce flakiness of tests involving actual sleeps (round 2)

[dbjorge]

Resolves continuing issues like https://app.circleci.com/pipelines/github/dequelabs/axe-core/7418/workflows/a3f6cd97-f7f7-4093-99ae-3b7b6e8f5934/jobs/79830

[Copilot]

Pull Request Overview

This PR addresses flakiness in performance timer tests by adding tolerance for browser sleep timing inconsistencies. The tests were failing when actual sleep durations were slightly less than expected due to browser main thread synchronization behavior.

• Adds a tolerance constant for animation frame timing variations
• Updates all assertions that check for minimum sleep duration to account for timing variations

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