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.

Confronta le linee guida unificate per i test E2E con le linee guida specifiche per dispositivi mobili ed estensioni menzionate nell’issue. Consolida le indicazioni applicabili a entrambe le piattaforme, incluso evitare ritardi arbitrari, e assicurati che la documentazione risultante copra le lacune identificate senza duplicare le indicazioni specifiche della piattaforma.

Scritto dal modello di indicizzazione a partire dal testo della issue.