Refactor device template tests to separate DeviceTemplateTests project

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This PR refactors the device-specific template tests from the Microsoft.Maui.IntegrationTests project into a new separate project called Microsoft.Maui.DeviceTemplateTests to improve test isolation and reliability.

Changes Made

New Project Structure

• Created Microsoft.Maui.DeviceTemplateTests project with XUnit test framework
• Moved the following test classes from IntegrationTests:
  • AppleTemplateTests - iOS/macOS template deployment tests
  • AndroidTemplateTests - Android template deployment tests
  • AOTTemplateTest - AOT compilation tests for iOS/macOS

Framework Migration

• Converted all tests from NUnit to XUnit framework:
  • [Test] → [Fact] / [Theory]
  • [TestCase(...)] → [InlineData(...)]
  • [SetUp] / [TearDown] → Constructor / IDisposable pattern
  • Assert.IsTrue() → Assert.True()
  • Assert.AreEqual() → Assert.Equal()

Dependencies and Utilities

• Copied all required base classes (BaseBuildTest, BaseTemplateTests)
• Copied platform-specific utilities (Android/, Apple/ folders)
• Copied XHarness utilities and other required utility classes
• Copied test resources and embedded files
• Updated all namespace references to use the new project namespace

Solution Integration

• Updated all relevant solution files to include the new project:
  • Microsoft.Maui.sln
  • Microsoft.Maui-dev.sln
  • Microsoft.Maui-vscode.sln
  • Microsoft.Maui-mac.slnf
  • Microsoft.Maui-windows.slnf
  • Microsoft.Maui.LegacyControlGallery.sln

Benefits

• Improved Isolation: Device template tests can now run independently from other integration tests
• Better Reliability: Separate project reduces failures affecting unrelated test suites
• Enhanced CI/CD: Better suited for Helix and adhoc test runs that were previously unreliable
• Modern Framework: XUnit provides better tooling and CI integration compared to NUnit
• Cleaner Architecture: Clear separation between general integration tests and device-specific tests

Testing

• ✅ New DeviceTemplateTests project builds successfully
• ✅ Original IntegrationTests project still builds without moved classes
• ✅ XUnit test discovery and execution working properly
• ✅ All solution files updated and building correctly

The device-specific tests themselves are expected to fail in environments without iOS/Android development tools, but the test framework and project structure are verified to work correctly.

Fixes #30209.

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