Update ExpressionObserverTests_DataValidation.cs

fix Validation_Plugins_Send_Correct_Notifications to non 'en-us' CultureInfo.

tests/Avalonia.Markup.UnitTests/Data/ExpressionObserverTests_DataValidation.cs

@@ -78,6 +78,10 @@ namespace Avalonia.Markup.UnitTests.Data
             var data = new IndeiTest();
             var observer = new ExpressionObserver(data, nameof(data.MustBePositive), true);
             var result = new List<object>();
+            
+            var errmsg = string.Empty;
+            try { typeof(IndeiTest).GetProperty(nameof(IndeiTest.MustBePositive)).SetValue(data, "foo"); }
+            catch(Exception e) { errmsg = e.Message; }
 
             observer.Subscribe(x => result.Add(x));
             observer.SetValue(5);
@@ -99,7 +103,7 @@ namespace Avalonia.Markup.UnitTests.Data
 
                 // Exception is thrown by trying to set value to "foo".
                 new BindingNotification(
-                    new ArgumentException("Object of type 'System.String' cannot be converted to type 'System.Int32'."),
+                    new ArgumentException(errmsg),
                     BindingErrorType.DataValidationError),
 
                 // Value is set then validation is updated.