Resolve some build warnings

Author: Sergio0694

tests/CommunityToolkit.HighPerformance.UnitTests/Extensions/Test_SpinLockExtensions.cs

@@ -41,7 +41,7 @@ public void Test_ArrayExtensions_Ref()
 
         int sum = 0;
 
-        Parallel.For(0, 1000, i =>
+        _ = Parallel.For(0, 1000, i =>
         {
             for (int j = 0; j < 10; j++)
             {
@@ -54,7 +54,6 @@ public void Test_ArrayExtensions_Ref()
 
         Assert.AreEqual(sum, 1000 * 10);
     }
-#endif
 
     /// <summary>
     /// A dummy model that owns a <see cref="SpinLock"/> object.
@@ -63,4 +62,5 @@ private sealed class SpinLockOwner
     {
         public SpinLock Lock;
     }
+#endif
 }

tests/CommunityToolkit.Mvvm.UnitTests/Test_ObservablePropertyAttribute.cs

@@ -924,12 +924,12 @@ partial class ModelWithMultipleGenericParameters<T, U> : ObservableObject, IValu
         private bool value;
 
         [ObservableProperty]
-        private T tValue;
+        private T? tValue;
 
         [ObservableProperty]
-        private U uValue;
+        private U? uValue;
 
         [ObservableProperty]
-        private List<T> list;
+        private List<T>? list;
     }
 }