This allows us to disable test output, which performs expensive assert
tracking. This was making our imported tests run significantly slower
than tests run via `WPT.sh`.
Formatting the output ourselves also allows us to remove unnecessary
information from the test output.
This commit also rebaselines all existing imported WPT tests to follow
the new format.
This change adds handling for the “Determine Child Nodes” substep at
https://w3c.github.io/accname/#comp_name_from_content_find_child in the
“Accessible Name and Description Computation” spec. Specifically, it
adds handling for the “If the current node has an attached shadow root”
and “if the current node is a slot with assigned nodes” conditions.
Otherwise, without this change, AT users don’t hear the expected
accessible names in cases where the content for which an accessible name
being computed is in a shadow root or slot element.
