Fix flaky TestFragmenter

[SungJin1212]

This PR changes the TestFragmenter to use require.ElementsMatch to compare slices without order.

Reason

In PlanFragmenter, since child fragment IDs are collected using a map (childFragmentIDs), the ChildIDs slice is non-deterministic.

childFragmentIDs := make(map[uint64]bool)
children := (*current).Children()

...

childIDs := make([]uint64, 0, len(childFragmentIDs))
for fragmentID := range childFragmentIDs {
	childIDs = append(childIDs, fragmentID)
}

...

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[yeya24]

Thanks for the fix.