Also backported tests from our stand-alone Json lib to tolerate subtle
differences between simdjson and our backend. The differences are
really minor:
1. simdjson tolerates invalid unicode codepoint escapes (\udd6a) whereas our
lib is more restrictive.
2. simdjson has a maximum nesting level of 1024 and our lib has 512.
In either case these subtleties are not a problem in practice -- just that the
tests needed to be updated to tolerate these differences.
