Fix replaceInstances messing up GetChildren ordering (#1265)

Fixes #1257. replaceInstances now preserves GetChildren ordering.

I also had the unit tests unreliably say that Packages wasn't a member
of ReplicatedStorage so I added WaitForChild and tests run 100% for me
now.

plugin/src/orderSwaps.spec.lua

@@ -0,0 +1,57 @@
+return function()
+	local orderSwaps = require(script.Parent.orderSwaps)
+
+	it("orders same-named siblings by their original GetChildren order", function()
+		local parent = Instance.new("Model")
+		local a1 = Instance.new("Part")
+		a1.Name = "a"
+		a1.Parent = parent
+		local a2 = Instance.new("Part")
+		a2.Name = "a"
+		a2.Parent = parent
+		local a3 = Instance.new("Part")
+		a3.Name = "a"
+		a3.Parent = parent
+
+		-- Input deliberately out of sibling order.
+		-- orderSwaps must restore the GetChildren() order.
+		local ordered = orderSwaps({
+			{ id = "3", oldInstance = a3 },
+			{ id = "1", oldInstance = a1 },
+			{ id = "2", oldInstance = a2 },
+		})
+
+		expect(ordered[1].oldInstance).to.equal(a1)
+		expect(ordered[2].oldInstance).to.equal(a2)
+		expect(ordered[3].oldInstance).to.equal(a3)
+	end)
+
+	it("orders ancestors before descendants", function()
+		local root = Instance.new("Model")
+		local child = Instance.new("Folder")
+		child.Parent = root
+		local grandchild = Instance.new("Part")
+		grandchild.Parent = child
+
+		local ordered = orderSwaps({
+			{ id = "grandchild", oldInstance = grandchild },
+			{ id = "child", oldInstance = child },
+			{ id = "root", oldInstance = root },
+		})
+
+		expect(ordered[1].oldInstance).to.equal(root)
+		expect(ordered[2].oldInstance).to.equal(child)
+		expect(ordered[3].oldInstance).to.equal(grandchild)
+	end)
+
+	it("returns a single swap unchanged", function()
+		local part = Instance.new("Part")
+
+		local ordered = orderSwaps({
+			{ id = "1", oldInstance = part },
+		})
+
+		expect(#ordered).to.equal(1)
+		expect(ordered[1].oldInstance).to.equal(part)
+	end)
+end