ZTS: snapshot_018_pos.ksh add extra margin

[behlendorf]

Motivation and Context

https://github.com/openzfs/zfs/actions/runs/24764467025/job/72455377060?pr=18445

...
  08:26:05.98 SUCCESS: zfs mount testpool/testfs
  08:26:05.99 SUCCESS: eval [[ 1776846365 == 1776846365 ]]
  08:26:05.99 SUCCESS: eval [[ 1776846365889999399 == 1776846365889999399 ]]
  08:26:06.00 SUCCESS: eval [[ 1776846365 ==  1776846365 ]]
  08:26:06.02 SUCCESS: zfs destroy testpool/testfs@v1
  08:26:06.03 ERROR: eval [[ 1776846365 -ge 1776846366 ]] exited 1

Description

The date(1) command and snapshot timestamps use different clock sources which can result in a small discrepancy. This can cause the test the incorrectly fail. To avoid this, add a brief delay to the test case to allow for minor skew.

How Has This Been Tested?

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)
• [q] 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)

[Copilot]

Pull request overview

This PR addresses a ZTS flake in snapshot_018_pos.ksh where snapshot timestamps can be slightly behind date(1) due to differing clock sources, causing occasional failures in snapshots_changed time comparisons.

Changes:

• Insert a 1-second delay after capturing curr_time and before snapshot/destroy operations to tolerate minor timestamp skew.
• Apply the delay consistently across all the test’s curr_time-based timestamp assertions.

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