Export and log the DOM on failure

# Export and log the DOM on failure

## Context

When a Selenium tests failed, ARA will show the failing step, the screenshot and video recording and, eventually, the Javascript errors.

But if an HTML node can't be reached or selected, we can't absolutely explain it using the screenshot.

## Expected

When a Selenium test failed, then we should also extract the DOM and put it in the tests results, just like the screenshot and video recording.

Trace how failed Selenium tests currently add screenshots and video recordings to the test results. Add the DOM export and logging alongside those artifacts, then verify that a failed test result contains the DOM as well as the existing evidence.

Written by the indexing model from the issue text.