Split out sharable testing for non-element thing

[mattleibow]

Description of Change

This PR extracts the core idea of a "gallery" and makes it usable for non-one-element tests. I will be adding some input transparency tests and I can leverage this a fair bit.

An example usgae:

internal class InputTransparencyGalleryPage : CoreGalleryBasePage
{
    protected override void Build()
    {
        Add(Test.Thing1, new ViewContainer(<test here>));
        Add(Test.Thing2, new ViewContainer(<test here>));
    }
}

public class InputTransparencyGalleryTests : CoreGalleryBasePageTest
{
    const string ButtonGallery = "* marked:'Input Transparency Gallery'";

    public InputTransparencyGalleryTests(TestDevice device)
        : base(device)
    {
    }

    protected override void NavigateToGallery() =>
        App.NavigateToGallery(ButtonGallery);

    [Test]
    public void Thing1()
    {
        // ui test code
    }
    [Test]
    public void Thing2()
    {
        // ui test code
    }
}

[mattleibow]

/rebase

[mattleibow]

UI tests failures are unrelated and are also failing on main:

[mattleibow]

All fixed!!!