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.
...when running in test mode. This cuts down on the time it takes to run
the imported WPT tests, and you can still get the full error by opening
tests in the browser.
