ZTS: Adjust mmp_on_uberblocks threshold

[behlendorf]

Motivation and Context

Further reduce false positives in the CI. For example,

https://github.com/openzfs/zfs/actions/runs/22623131399/job/65552855503?pr=18277

Description

Decrease the number of required uberblock blocks write slightly due to observed variation when running in the CI.

How Has This Been Tested?

Updated value based on the lower value I've observed when looking at the CI results. Will be tested by the CI.

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

Adjusts the ZFS Test Suite’s MMP uberblock sequencing threshold to reduce CI flakiness/false positives when validating mmp_seq progression under MULTIHOST_INTERVAL tuning.

Changes:

• Lowers MIN_SEQ_VALUES from 10 to 8 in the mmp_on_uberblocks functional test to better match observed CI variability.

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