Add -v/--verbose support to JavaScript test runners so they print each
test file immediately before running it. This makes hard crashes and
process traps easy to map back to the file that triggered them.
This removes the use of StringBuilder::OutputType (which was ByteString,
and only used by the JSON classes). And it removes the StringBuilder
template parameter from the serialization methods; this was only ever
used with StringBuilder, so a template is pretty overkill here.
