Adds Task.CompletedTask await in test method

Adds `await Task.CompletedTask;` within the `SchemeFromVariant` test method.

Author: Keboo

tests/MaterialColorUtilities.Tests/ContrastConstraintTests.cs

@@ -352,6 +352,7 @@ public async Task Scheme_Correctness_All()
             }
 
             // If we got here, constraints passed for this (variant, seed, CL, dark) tuple.
+            await Task.CompletedTask;
         }
     }
 }