Consolidate E2E test documentation between platforms

We currently have one unified e2e testing guidelines document, and an additional document for mobile and extension specific guidelines. Most of these platform-specific guidelines are not really platform-specific, and could be adapted to apply to both platforms.

This would fill in some gaps currently in our documentation as well. For example, the mobile E2E test guidelines don't suggest anywhere that we should avoid arbitrary delays, but the extension guidelines do. Arbitrary delays are an anti-pattern that results in slower and more flaky tests, and should be used as a last resort on both platforms, not just extension.

Comparez les directives unifiées de test E2E avec les directives spécifiques aux appareils mobiles et aux extensions mentionnées dans l’issue. Regroupez les recommandations applicables aux deux plateformes, notamment l’évitement des délais arbitraires, et veillez à ce que la documentation résultante couvre les lacunes identifiées sans dupliquer les recommandations spécifiques à chaque plateforme.