deqp_runner: Add a flag to save log files for xfails, too.

For CI runs we like to reduce the log file count to be uploaded, but for devs often it's nice to just keep them all for review.

The consistency of using should_save_logs() here means it also saves dEQP caselists for xfails (in case it's an order-dependent failure), and also dEQP flakes save QPAs.

Fixes: #22 (closed)