zts: add a debug option to get full test output

The test runner accumulates output from individual tests, then writes it
to the log at the end. If a test hangs or crashes the system half way
through, we get no insight into how it got to where it did.

This adds a -D option for "debug". When set, all test output is written
to stdout.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Akash B <akash-b@hpe.com>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #16096