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.

Compara las directrices unificadas de pruebas E2E con las directrices específicas para móviles y extensiones mencionadas en el issue. Consolida las indicaciones aplicables a ambas plataformas, incluida la de evitar retrasos arbitrarios, y asegúrate de que la documentación resultante cubra las carencias identificadas sin duplicar las indicaciones específicas de cada plataforma.

Escrito por el modelo de indexación a partir del texto del issue.