Capture screenshots on failures

Just learned about this api: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/captureVisibleTab though, capturing a screenshot of the test area may require something else like: https://github.com/NullVoxPopuli/limber/blob/main/frontend/app/components/limber/copy-menu.gts#L21

Would be great to include a screenshot of the test area in failed assertions, just to provide more information and allow folks to maybe see whan happened before rerunning tests with added pause/debugger/logs/etc