Changes test method to void return

Updates the `Scheme_Correctness_All` method signature from `async Task` to `void`.

Author: Keboo

tests/MaterialColorUtilities.Tests/ContrastConstraintTests.cs

@@ -334,7 +334,7 @@ private static DynamicScheme SchemeFromVariant(Variant variant, Hct source, bool
 
     [Test]
     [DisplayName("scheme_correctness_test: all variants & contrast levels satisfy constraints")]
-    public async Task Scheme_Correctness_All()
+    public void Scheme_Correctness_All()
     {
         var constraints = BuildConstraints();