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.
The spec just says to follow "most backwards-compatible, then shortest"
when serializing these (and it does so in a very hand-wavy fashion).
By omitting some keywords when they are implied, we end up matching
other engines and pass a bunch of WPT tests.
