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.

Compare the unified E2E testing guidelines with the mobile and extension-specific guidelines mentioned in the issue. Consolidate guidance that applies to both platforms, including avoiding arbitrary delays, and ensure the resulting documentation covers the identified gaps without duplicating platform-specific advice.

Written by the indexing model from the issue text.