zts-report: don't crash on non-UTF-8 chars in the log (#16497)

The report generator expects the log to be clean and tidy UTF-8. That
can be a problem if you use some of the verbose/debug test runner
options, which sends all sorts of weird output from arbitrary programs
to the log.

This just makes Python a little more relaxed about such things. It
shouldn't matter in practice, as those lines didn't match the test
result regex anyway, and are discarded immediately.

Sponsored-by: https://despairlabs.com/sponsor/

Signed-off-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>