ZTS: Add slow_vdev_degraded_sit_out retry

[behlendorf]

Motivation and Context

Keep the CI green. We see this occasional false positive in the CI which may distract from other real issues.

https://github.com/openzfs/zfs/actions/runs/19481983821/job/55755445615?pr=17941

I'm not thrilled about the need for this, but given that the test itself isn't entirely deterministic a retry seems like a reasonable compromise.

Description

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.

How Has This Been Tested?

Locally tested by manually injecting random percentage of failed sit outs to test to retry code.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[Copilot]

Pull request overview

This PR adds retry logic to the slow_vdev_degraded_sit_out ZFS test to handle non-deterministic behavior observed in CI, particularly with draid3 vdev types that don't always trigger a sit-out condition.

Key changes:

• Converts the test loop from a foreach-style iteration over raid types to an indexed loop to enable retries
• Adds conditional retry logic (up to 3 attempts) when a vdev doesn't sit out as expected
• Removes the unconditional assertion that sit_out must be "on" after I/O operations

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