A "namespace prefix map", see:
https://w3c.github.io/DOM-Parsing/#the-namespace-prefix-map
Is meant to also hold null namespaces:
> where namespaceURI values are the map's unique keys
> (which can include the null value representing no namespace)
Which we previously neglected. This resulted in a crash for
the updated WPT test.
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.
While the code that did this referred to the HTML spec, other browsers
appear not to have this behavior when parsing XML, and it breaks a WPT
subtest.
This change does not appear to break any tests, and fixes 1 WPT subtest.
