Fix a couple of CA warnings.

Author: grokys

test/GitHub.App.UnitTests/ViewModels/GitHubPane/PullRequestListViewModelTests.cs

@@ -21,7 +21,7 @@ public async Task OpenItem_Navigates_To_Correct_Fork_Url()
             var uri = (Uri)null;
             target.NavigationRequested.Subscribe(x => uri = x);
 
-            target.OpenItem.Execute(target.Items[1]);
+            await target.OpenItem.Execute(target.Items[1]);
 
             Assert.That(uri, Is.EqualTo(new Uri("github://pane/owner/name/pull/2")));
         }

test/Helpers/ReactiveTestHelper.cs

@@ -29,7 +29,7 @@ public static bool CanExecute<TParam, TResult>(this ReactiveCommand<TParam, TRes
     }
 }
 
-public class UserErrorResult : IDisposable
+public sealed class UserErrorResult : IDisposable
 {
     readonly IDisposable handlerOverride;