In the Smart Window tests it can be quite confusing to query select and
find an element that you need access to. This commit creates a new
testing primitive that makes it easy to cut across shadow root
boundaries and query select through open or closed shadow roots. Many
Smart Window tests confuse the behavior of being blocked by a shadow
root, and being blocked by content being in a content process. This
reduces the potential confusion and complexity around selecting these
components, as frequently tests are using SpecialPowers.spawn to
unnecessarily complicate tests, when it's really just hidden within the
shadow dom.
Differential Revision: https://phabricator.services.mozilla.com/D297913
Greg Tatum
cff6e11198