Ensures single click event invocation

Adds a wait to ensure that the click event is only invoked a single time after a click.

tests/MaterialDesignThemes.UITests/WPF/SplitButtons/SplitButtonTests.cs

@@ -97,9 +97,11 @@ public async Task SplitButton_WithButtonInPopup_CanBeInvoked()
         await Task.Delay(50, TestContext.Current!.CancellationToken);
 
         // Assert
-        var invocations = await clickEvent.GetInvocations();
-        await Assert.That(invocations).HasSingleItem();
-
+        await Wait.For(async () =>
+        {
+            var invocations = await clickEvent.GetInvocations();
+            await Assert.That(invocations).HasSingleItem();
+        });
         recorder.Success();
     }