Fix MVVM Toolkit source generator tests

Author: Sergio0694

UnitTests/UnitTests.SourceGenerators/Test_SourceGeneratorsDiagnostics.cs

@@ -9,6 +9,7 @@
 using System.Linq;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.Toolkit.Mvvm.ComponentModel;
 using Microsoft.Toolkit.Mvvm.SourceGenerators;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 
@@ -249,6 +250,7 @@ public partial class SampleViewModel
         private void VerifyGeneratedDiagnostics<TGenerator>(string source, params string[] diagnosticsIds)
             where TGenerator : class, ISourceGenerator, new()
         {
+            Type observableObjectType = typeof(ObservableObject);
             Type validationAttributeType = typeof(ValidationAttribute);
 
             SyntaxTree syntaxTree = CSharpSyntaxTree.ParseText(source);
@@ -271,6 +273,7 @@ from assembly in AppDomain.CurrentDomain.GetAssemblies()
 
             Assert.IsTrue(resultingIds.SetEquals(diagnosticsIds));
 
+            GC.KeepAlive(observableObjectType);
             GC.KeepAlive(validationAttributeType);
         }
     }

UnitTests/UnitTests.SourceGenerators/UnitTests.SourceGenerators.csproj

@@ -15,6 +15,7 @@
 
   <ItemGroup>
     <ProjectReference Include="..\..\Microsoft.Toolkit.Mvvm.SourceGenerators\Microsoft.Toolkit.Mvvm.SourceGenerators.csproj" />
+    <ProjectReference Include="..\..\Microsoft.Toolkit.Mvvm\Microsoft.Toolkit.Mvvm.csproj" />
   </ItemGroup>
 
 </Project>