Fix 2 trivial tech issues

Author: craigfowler

Tests/CSF.Screenplay.Tests/ActorTests.cs

@@ -109,12 +109,12 @@ public void PerformAsyncWithGenericResultShouldThrowIfCalledAfterActorIsDisposed
         Assert.That(() => ((ICanPerform) sut).PerformAsync(performable), Throws.InstanceOf<ObjectDisposedException>());
     }
 
-    public class DisposableAbility1 : IDisposable
+    public sealed class DisposableAbility1 : IDisposable
     {
         public virtual void Dispose() {}
     }
 
-    public class DisposableAbility2 : IDisposable
+    public sealed class DisposableAbility2 : IDisposable
     {
         public virtual void Dispose() {}
     }