Previously, some fuzzers were generating an excessive amount of debug
logging. This change explicitly disables debug logging for all fuzzers.
This allows higher test throughput and makes the logs easier to read
when fuzzing locally.
Instead of trying to create a Window and a Document, and use those to
create a ParsingContext, just use the JS::Realm only constructor to make
sure that bindings are stashed on the main thread VM's realm.
