Add a few test for the hierarchySort function

[crazytonyli]

Issue

The hierarchySort function is extremely slow. It only contains three statements, but every single one of them are O(n^2) functions.

WordPress-iOS/WordPress/Classes/Extensions/Array+Page.swift

Lines 82 to 89 in 6d6f1ca

	func hierarchySort() -> [Element] {
	return map {
	$0.hasVisibleParent = !$1.containsPage(for: $0.parentID?.intValue)
	return $0
	}
	.sort()
	.hierachyIndexes()
	}

Test in the app

The issue can be observed on Pages List. Due to an existing bug #21813, Pages List on a site that has lots of pages can't be displayed. If you want to see this issue in a real app, check out this branch, launch the app from Xcode to an iOS device, choose Field Guide (or any site that have 500+ pages), go to pages list, and scroll through the pages.

From my test, the issue is so bad that it's miserable to scroll though Field Guide pages. For a site with 600 pages, scrolling through the list would still gets jerky during fetching the pages.

This PR is not a fix

I'm planning to fix this issue, but this PR is not it. This PR adds some basic unit tests for this function and I'll work on addressing the issue separately.

Regression Notes

N/A

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr22048-e149f6b
	Version	23.7
	Bundle ID	org.wordpress.alpha
	Commit	e149f6b
	App Center Build	WPiOS - One-Offs #7853

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[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr22048-e149f6b
	Version	23.7
	Bundle ID	com.jetpack.alpha
	Commit	e149f6b
	App Center Build	jetpack-installable-builds #6878

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