Add built-in theme tests and fix several bugs found by them

[maxkatz6]

What does the pull request do?

Adds a new Avalonia.Themes.UnitTests project that validates the built-in Fluent and Simple themes, and fixes the theme issues these tests found.

What is the current behavior?

Before this PR, built-in themes were not covered by any tests. Several types of issues existed unnoticed:

• DynamicResource targetting undefined resources
• Brushes/colors being hardcoded in the setter or template parts
• Several XAML files were compiled as public (doubling IL output, as we merge XAML files)
• StringCalendarWeekNumberHeader apparently was still ignored as a resource

What is the updated/expected behavior with this PR?

New tests:

• Theme class has a ctor(IServiceProvider) and can be reached via avares: (which we technically always supported).
• All XAML files should be internal (except entry point).
• Every theme dictionary defines the same resources.
• No global styles are defined, only ControlTheme with nested styles.
• Every built-in templated control has a ControlTheme (with some known exceptions).
• Every [TemplatePart] is defined in the template.
• Every {DynamicResource} references a defined key.
• No brush, color or box shadow is hardcoded in a setter or a template

How was the solution implemented (if it's not obvious)?

All tests are defined in abstract classes with specific implementations per each theme:

[Trait("Category", "FluentTheme")]
public class FluentControlThemeTests() : ControlThemeTests(typeof(FluentTheme));
[Trait("Category", "SimpleTheme")]
public class SimpleControlThemeTests() : ControlThemeTests(typeof(SimpleTheme));

public abstract class ControlThemeTests(Type typeEntryPoint) : ThemeTestBase(typeEntryPoint)
{
}

With an idea to later add Fluent2 to the same test project.

Breaking changes

None

Disclamer: This PR was written by hand, without an AI. Code might be better or worse depending on who reads.
But it still was reviewed by Opus.

[avaloniaui-bot]

You can test this PR using the following package version. 12.2.999-cibuild0069865-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[MrJul]

Good set of tests!
A small nitpick, otherwise LGTM.

[MrJul]

Nit: add an Assert.NotEmpty(templatedControls) before, to make sure the test doesn't pass for the wrong reason.