ZTS: Add slow_vdev_degraded_sit_out retry

While not common the draid3 vdev type has been observed to
not always sit out a vdev when run in the CI. To prevent
continued false positives allow the test to be retried up
to three times before considering it a failure.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #18003