Add NonParallelizable attribute to tests (#4161)

## Pull Request Overview

This PR adds the `[NonParallelizable]` attribute to UI test classes to
prevent test failures caused by concurrent access to static classes.
This is a targeted fix to address flaky test behavior in the ReactiveUI
test suite.

- Adds `[NonParallelizable]` attribute to test classes that access
static members
- Prevents race conditions during parallel test execution
- Improves test reliability and consistency

Author: glennawatson

src/ReactiveUI.Tests/Locator/DefaultViewLocatorTests.cs

@@ -9,6 +9,7 @@ namespace ReactiveUI.Tests;
 /// Tests for the default view locators.
 /// </summary>
 [TestFixture]
+[NonParallelizable]
 public partial class DefaultViewLocatorTests
 {
     /// <summary>