SL-18837: Make llsdserialize_test debug output conditional.

Move hexdump() and hexmix() stream formatters to new hexdump.h for potential
use by other tests.

In toPythonUsing() helper function, add a temp file to receive Python script
debug output, and direct debug output to that file. On test failure, dump the
contents of that file to the log.

Give NamedTempFile::peep() an optional target std::ostream; refactor
implementation as peep_via() that accepts a callable to process each text
line. Add operator<<() to stream the contents of a NamedTempFile object to
ostream -- but don't use that with LL_DEBUGS(), as it flattens the file
contents into a single log line. Instead add peep_log(), which streams each
individual text line to LL_DEBUGS().