Ensure running lighthouse with external emulation should work with full-page-screenshot

The potential impact of this may even be negligible, since grabbing the full page screenshot is the last thing Lighthouse does. Probably doesn't matter if the emulation isn't set back to what it was set to externally at the end of the test ... the calling code shouldn't be reusing the page, right? Lighthouse should be terminal? I'm Ron Burgundy?

ref #10910, https://github.com/GoogleChrome/lighthouse/pull/10716#discussion_r428943503

Start with the behavior described in this issue and compare it with ref #10910 and the discussion on pull request #10716. Trace how external emulation interacts with full-page-screenshot, then verify whether the emulation state is restored or whether the screenshot still completes correctly.

Written by the indexing model from the issue text.