Added UiTests (dotnet#12429)

Author: kubaflo

src/Controls/tests/TestCases.HostApp/Issues/Issue12429.xaml

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+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="Maui.Controls.Sample.Issues.Issue12429"
+             x:Name="Self"
+             Title="Issue12429">
+    <Grid RowDefinitions="Auto,*">
+        <HorizontalStackLayout Grid.Row="0">
+            <Button
+                Command="{Binding AddCommand}"
+                HorizontalOptions="Center"
+                AutomationId="button"
+                Text="Add" />
+        </HorizontalStackLayout>
+
+        <CollectionView
+            Grid.Row="1"
+            ItemsSource="{Binding ItemList}">
+
+            <CollectionView.Header>
+                <VerticalStackLayout>
+                    <VerticalStackLayout BindableLayout.ItemsSource="{Binding BindingContext.ItemListHeader, Source={x:Reference Self}}">
+                        <BindableLayout.ItemTemplate>
+                            <DataTemplate>
+                                <Label Padding="10" Text="{Binding .}" />
+                            </DataTemplate>
+                        </BindableLayout.ItemTemplate>
+                    </VerticalStackLayout>
+                    <BoxView
+                        Margin="0,5"
+                        HeightRequest="2"
+                        Color="Black" />
+                </VerticalStackLayout>
+            </CollectionView.Header>
+
+            <CollectionView.ItemTemplate>
+                <DataTemplate>
+                    <Label Padding="10" Text="{Binding .}" />
+                </DataTemplate>
+            </CollectionView.ItemTemplate>
+        </CollectionView>
+    </Grid>
+</ContentPage>

src/Controls/tests/TestCases.HostApp/Issues/Issue12429.xaml.cs

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+using System.Collections.ObjectModel;
+using Microsoft.Maui.Controls;
+using Microsoft.Maui.Controls.Xaml;
+
+namespace Maui.Controls.Sample.Issues;
+
+[XamlCompilation(XamlCompilationOptions.Compile)]
+[Issue(IssueTracker.Github, 12429, "[iOS] CollectionView Items display issue when Header is resized", PlatformAffected.iOS)]
+
+public partial class Issue12429 : ContentPage
+{
+	public ObservableCollection<string> ItemList { get; }
+	public ObservableCollection<string> ItemListHeader { get; }
+
+	public Command AddCommand => new(() => ItemListHeader.Add($"HeaderItem{ItemListHeader.Count + 1}"));
+
+	public Issue12429()
+	{
+		InitializeComponent();
+		ItemList = new() { "Item1", "Item2", "Itme3" };
+		ItemListHeader = new() { "HeaderItem1" };
+		BindingContext = this;
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue12429.cs

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+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.AppiumTests.Issues;
+
+public class Issue12429 : _IssuesUITest
+{
+	public override string Issue => "[iOS] CollectionView Items display issue when Header is resized";
+
+	public Issue12429(TestDevice device) : base(device)
+	{ }
+
+    [Test]
+	public void HeaderShouldNotCollapseWithItems()
+	{
+		App.WaitForElement("button");
+		App.Click("button");
+		App.Click("button");
+		App.Click("button");
+
+		//The test passes of header has 4 elements that don't overlap with the collection view's items
+		VerifyScreenshot();
+	}
+}